Every supports the notion of equality, with the following rules:

Note that a consequence of the above is that, given  A and  B where A and B are not related by or , for every pair of values a from A and b from B, a != b.

On every datatype, the operation Equal is defined in terms of the equality property of the : for any values a, b drawn from the , Equal(a,b) is true if a = b, and false otherwise.

An order relation on a is a mathematical relation that imposes a or a on the members of the .

A , and hence a datatype, is said to be ordered if there exists an defined for that .

A partial order is an that is irreflexive, antisymmetric and transitive.

A has the following properties:

The notation a <> b is used to indicate the case when a != b and neither a < b nor b < a

A total order is an such that for no a and b is it the case that a <> b.

A has all of the properties specified above for , plus the following property:

A value u in an   U is said to be an inclusive upper bound of a  V (where V is a subset of U) if for all v in V, u >= v.

A value u in an   U is said to be an exclusive upper bound of a  V (where V is a subset of U) if for all v in V, u > v.

A value l in an   L is said to be an inclusive lower bound of a  V (where V is a subset of L) if for all v in V, l <= v.

A value l in an   L is said to be an exclusive lower bound of a  V (where V is a subset of L) if for all v in V, l < v.

A datatype is bounded if its has either an or an and either an and an .

Every has associated with it the concept of cardinality. Some s are finite, some are countably infinite while still others could conceivably be uncountably infinite (although no defined by this specification is uncountable infinite). A datatype is said to have the cardinality of its .

It is sometimes useful to categorize s (and hence, datatypes) as to their cardinality. There are two significant cases:

A datatype is said to be numeric if its values are conceptually quantities (in some mathematical number system).

A datatype whose values are not is said to be non-numeric.

length is the number of units of length, where units of length varies depending on the type that is being from. The value of length  be a .

For and datatypes from , length is measured in units of characters as defined in . For and and datatypes from them, length is measured in octets (8 bits) of binary data. For datatypes by , length is measured in number of list items.

minLength is the minimum number of units of length, where units of length varies depending on the type that is being from. The value of minLength   be a .

For and datatypes from , minLength is measured in units of characters as defined in . For and and datatypes from them, minLength is measured in octets (8 bits) of binary data. For datatypes by , minLength is measured in number of list items.

maxLength is the maximum number of units of length, where units of length varies depending on the type that is being from. The value of maxLength   be a .

For and datatypes from , maxLength is measured in units of characters as defined in . For and and datatypes from them, maxLength is measured in octets (8 bits) of binary data. For datatypes by , maxLength is measured in number of list items.

pattern is a constraint on the of a datatype which is achieved by constraining the to literals which match a specific pattern. The value of pattern   be a .

enumeration constrains the to a specified set of values.

enumeration does not impose an order relation on the it creates; the value of the property of the datatype remains that of the datatype from which it is .

whiteSpace constrains the of types from such that the various behaviors specified in Attribute Value Normalization in are realized. The value of whiteSpace must be one of {preserve, replace, collapse}.

whiteSpace is applicable to all and datatypes. For all datatypes other than (and types by from it) the value of whiteSpace is collapse and cannot be changed by a schema author; for the value of whiteSpace is preserve; for any type by from the value of whiteSpace can be any of the three legal values. For all datatypes by the value of whiteSpace is collapse and cannot be changed by a schema author. For all datatypes by  whiteSpace does not apply directly; however, the normalization behavior of types is controlled by the value of whiteSpace on that one of the against which the is successfully validated.

maxInclusive is the of the for a datatype with the property. The value of maxInclusive  be in the of the .

maxExclusive is the of the for a datatype with the property. The value of maxExclusive   be in the of the .

minInclusive is the of the for a datatype with the property. The value of minInclusive   be in the of the .

minExclusive is the of the for a datatype with the property. The value of minExclusive  be in the of the .

totalDigits is the maximum number of digits in values of datatypes from . The value of totalDigits  be a .

fractionDigits is the maximum number of digits in the fractional part of values of datatypes from . The value of fractionDigits   be a .

datatypes can be either or . The of an datatype is a set of "atomic" values, which for the purposes of this specification, are not further decomposable. The of an datatype is a set of literals whose internal structure is specific to the datatype in question.

Several type systems (such as the one described in ) treat datatypes as special cases of the more general notions of aggregate or collection datatypes.

datatypes are always . The of a datatype is a set of finite-length sequences of values. The of a datatype is a set of literals whose internal structure is a white space separated sequence of literals of the datatype of the items in the (where whitespace es S in ).

The datatype that participates in the definition of a datatype is known as the itemType of that datatype.

A datatype can be from an datatype whose allows whitespace (such as or ). In such a case, regardless of the input, list items will be separated at whitespace boundaries.

When a datatype is from a datatype, the following s apply:

For each of , and , the unit of length is measured in number of list items. The value of is fixed to the value collapse.

The for the datatype is defined as the lexical form in which each item in the has the canonical lexical representation of its .

The and of a datatype are the union of the s and s of its . datatypes are always . Currently, there are no   datatypes.

Any number (greater than 1) of or s can participate in a type.

The datatypes that participate in the definition of a datatype are known as the memberTypes of that datatype.

The order in which the are specified in the definition (that is, the order of the <simpleType> children of the <union> element, or the order of the s in the memberTypes attribute) is significant. During validation, an element or attribute's value is validated against the in the order in which they appear in the definition until a match is found. The evaluation order can be overridden with the use of xsi:type. See and for more details.

The for a datatype is defined as the lexical form in which the values have the canonical lexical representation of the appropriate .

A datatype is said to be by restriction from another datatype values for one or more s are specified that serve to constrain its and/or its to a subset of those of its .

Every datatype that is by restriction is defined in terms of an existing datatype, referred to as its base type. base types can be either or .

A datatype can be from another datatype (its ) by creating a that consists of a finite-length sequence of values of its .

One datatype can be from one or more datatypes by ing their s and, consequently, their s.