A value space, and hence a datatype, is said to be ordered if there exists an order relation defined for that value space. Order relations have the following rules:

There may exist several possible order relations for a given value space. Additionally, there may exist multiple datatypes with the same value space. In such cases, each datatype will define a different order relation on the value space.

Some ordered value spaces, and hence some datatypes, are said to be bounded. A value space is bounded above if there exists a unique value U in the value space such that, for all values v in the value space, v ≤ U. The value U is said to be an upper bound of the value space. A value space is bounded below if there exists a unique value L in the space such that, for all values v in the value space, L ≤ v. The value L is then said to be a lower bound of the value space.

A datatype is bounded if its value space has both an upper and a lower bound.

Every value space has associated with it the concept of cardinality. Some value spaces are finite, some are countably infinite while still others are uncountably infinite. A datatype is said to have the cardinality of its value space. It is sometimes useful to categorize value spaces ( and hence, datatypes) as to their cardinality, there are three significant cases:

Every conceptually finite value space is necessarily exact. No computational datatype is uncountably infinite.

The computational representation of a datatype may limit the degree to which values of the datatype can be distinguished. If every value in the value space of the conceptual datatype is distinguishable in the computational representation from every other value in the value space, then the datatype is said to be exact.

Certain mathematical datatypes with very large or infinite value spaces have representations which are said to be approximate in that multiple values in the conceptual value space map to single values in the value space of the representation. In this specification, all approximate datatypes have computational models which specify, via parametric values, a degree of approximation, that is, they require a certain minimum set of values of the mathematical datatype to be distinguishable in the computational datatype. Further, each value in the conceptual value space must be be capable of being represented in the representational value space within a certain distance i.e. the difference between the conceptual value and the representational value must not exceed some agreed upon value.

A datatype is said to be numeric if its values are conceptually quantities (in some mathematical number system). A datatype whose values do not have this property is said to be non-numeric.

For the datatype, length specifies the number of allowable characters in the string. For the datatype it specifies the length in bits. The value of the length facet must be a positive integer.

The maxlength facet indicates the maximum length, in characters, of a datatype for which the facet is not specified. For the datatype it specifies the maximum length in bits if the facet is not specified. The value of the maximum length facet must be a positive integer.

The datatypes defined in this specification are defined in terms of abstract value spaces and their properties as opposed to how values are lexically represented in XML instances. However, the lexical representation of values is of prime importance in many applications. Because of this importance, each definition includes a detailed description of its default . The lexical representation facet can be used to constrain the allowable representations, or literals, for values of a datatype. The meaning of the lexical representation facet depends on the datatype to which it is applied.

For example, for , values for the lexical representation facet are .

Presence of an enumeration facet constrains the value space of the datatype to the specified list. The enumeration facet can be applied to any datatype. No order or any other relationship is implied between the elements of the enumeration list.

The minAbsoluteValue facet specifies the minimum absolute value of the value space for generated datatypes whose basetype is . This facet (together with ) can be used to generate subtypes of which correspond to common floating point representations.

The maxAbsoluteValue facet specifies the maximum absolute value of the value space for generated datatypes whose basetype is . This facet (together with ) can be used to generate subtypes of which correspond to common floating point representations.

The maxInclusive facet determines the upper bound of the value space for a datatype with the property. The maximum value specified with this facet is inclusive in the sense that the value specified for the facet is itself included in the value space for the datatype.

The maxExclusive facet determines the upper bound of the value space for a datatype with the property. The maximum value specified with this facet is exclusive in the sense that the value specified for the facet is itself excluded from the value space for the datatype.

The minInclusive facet determines the lower bound of the value space for a datatype with the property. The minimum value specified with this facet is inclusive in the sense that the value specified for the facet is itself included in the value space for the datatype.

The minExclusive facet determines the lower bound of the value space for a datatype with the property. The minimum value specified with this facet is exclusive in the sense that the value specified for the facet is itself excluded from the value space for the datatype.

The precision facet, which only applies to the datatype refers to the total number of decimal digits in the number. Its value must be a positive integer.

The scale facet, which only applies to the datatype refers to the total number of decimal digits to the right of the decimal point. Its value must be a positive number less than or equal to the precision.

Every generated datatype is defined in terms of an existing datatype, referred to as the base type. Base types may be either primitive or generated.

In the example above, is referred to as a subtype of the base type . The value space of a subtype is a subset of the value space of the base type.

The string datatype has an optional constraining facet called . The value of this facet is a regular expression. Regular expression constraints are discussed in Appendix . If this facet is not present, there is no restriction on the lexical representation.

The string datatype has an optional constraining facet called . If length is specified we have a fixed length character string, whether length is measured in the number of characters in the string. If length is not specified we have a variable length character string. The value of the length facet must be a positive integer.

The string datatype has an optional constraining facet called . If maxlength is specified for a variable length string it represents an upper bound of the length of the string. The value of the maxlength facet must be a positive integer. Both and cannot be specified for the same datatype. The absolute maximum length of variable length character strings depends on the XML parser implementation.

The string datatype also has the following constraining facets:

Clearly, the effect of these constraining facets depends on the collating sequence used to define the property for strings.

An instance of a datatype that is defined as boolean can have the following legal lexical values {true, false}. The lexical representation is fixed and cannot be changed. The lexical representation facet is not supported.

real values have a single standard lexical representation consisting of a mantissa followed, optionally, by the character "E" followed by an exponent. The exponent must be an integer. The mantissa must be a decimal number. The representations for exponent and mantissa must follow the default lexical rules for integer and decimal numbers discussed above. If the "E" and the the following exponent are omitted, an exponent value of 1 is assumed. For example: -1E4, 1267.43233E12, 12.78E-2, 12.

The lexical representation for timeInstant is the representation CCYYMMDDThhmmss.sss where "CC" represents the century, "YY" the year, "MM" the month and "DD" the day. The letter "T" is the date/time separator and "hh", "mm", "ss.sss" represent hour, minute and second respectively. Note that this representation allows for fractional seconds.

This representation can be immediately followed by a "Z" to indicate Coordinated Universal Time. To indicate the time zone, i.e. the difference between the local time and Coordinated Universal Time, the difference immediately follows the time and consists of a sign, + or -, followed by hhmm.

For example, to indicate 1:20 pm on May the 31st, 1999 for Eastern Daylight Time which is 5 hours behind Coordinated Universal Time, one would write: 19990531T132000-0500.

The lexical representation for timeDuration is the representation CCYYMMDDThhmmss.sss, preceded by an optional sign (+ or -), where "CC" represents the number of centuries, "YY" the number of years, "MM" the number of months and "DD" the number of days. The letter "T" is the date/time separator and "hh", "mm", "ss.sss" represent number of hours, minutes and seconds respectively. Note that this representation allows for fractional seconds.

For example, to indicate a duration of 1 year, 2 months, 3 days, 10 hours, and 30 minutes, one would write: 00010203T103000.

Time periods, i.e. specific durations of time, can be represented by supplying two items of information: a start instant and a duration or a start instant and an end instant or an end instant and a duration.

The lexical representation for recurringInstant is the left truncated representation for . For example, if the century "CC" is omitted from the timeInstant representation it means a timeInstant that recurs every hundred years. Similarly, if "CCYY" is omitted it designates a time instant that recurs every year.

Every two character "unit" of the representation that is omitted is indicated by a single hyphen "-". For example, to indicate 1:20 pm on May the 31st every year, one would write write: --0531T132000-0500.

This required facet is used to specify the list of notations.

Decimal values have a single standard lexical representation. This consists of a string of digits separated by a period as a decimal indicator, in accordance with the scale and precision facets, with an optional leading sign to indicate a negative number. If the sign is omitted, "+" is assumed. Leading and trailing zeroes are optional. For example: -1.23, 12678967.543233, 100000.00.

Integer values have a single, standard lexical representation. This consists of a string of digits with an optional leading sign. If the sign is omitted, "+" is assumed. For example: -1, 0, 12678967543233, +100000.

Non-negative-integer values have a single, standard lexical representation. This consists of a string of digits with an optional leading "+" sign. If the sign is omitted, "+" is assumed. For example: 1, 0, 12678967543233, +100000.

positive-integer values have a single, standard lexical representation. This consists of a string of digits with an optional leading "+" sign. For example: 1, 12678967543233, +100000.

Non-positive-integer values have a single, standard lexical representation. This consists of a string of digits with a leading "-" sign. For example: -1, 0, -12678967543233, -100000.

negative-integer values have a single, standard lexical representation. This consists of a string of digits with a leading "-" sign. For example: -1, -12678967543233, -100000.

The lexical representation for date is the reduced (right truncated) lexical representation for : CCYYMMDD. For example, to indicate May the 31st, 1999, one would write: 19990531.

Left truncated representations can be used to represent recurring dates. If the CC is omitted it signifies a date that occurs every century. If the YY is omitted it signifies a date every year and so on. Every two character "unit" of the representation that is omitted is indicated by a single hyphen "-". For example, ---05 signifies the fifth day of every month.

The lexical representation for time is the left truncated lexical representation for : hhmmss.sss. For example, to indicate 1:20 pm for Eastern Daylight Time which is 5 hours behind Coordinated Universal Time, one would write: 132000-0500.