2. Terms and Definitions
This section is informative.
While some terms are defined in place, the following
definitions are used throughout this document. Familiarity
with the W3C XML 1.0 Recommendation [XML] is highly recommended.
a class of documents sharing a common abstract structure.
The ISO 8879 [SGML]
definition is as follows: "a class of documents having
similar characteristics; for example, journal, article,
technical manual, or memo. (4.102)"
the effective structure and constraints of a given document
type. The document model constitutes the abstract
representation of the physical or semantic structures of a
class of documents.
the markup vocabulary (ie., the gamut of element and
attribute names, notations, etc.) and grammar (ie., the
prescribed use of that vocabulary) as defined by a document
type definition (ie., a schema) The markup model is the
concrete representation in markup syntax of the document
model, and may be defined with varying levels of strict
conformity. The same document model may be expressed by a
variety of markup models.
document type definition (DTD)
a formal, machine-readable expression of the XML structure
and syntax rules to which a document instance of a specific
document type must conform; the schema type used in XML 1.0
to validate conformance of a document instance to its
declared document type. The same markup model may be
expressed by a variety of DTDs.
a DTD whose markup model represents the foundation of a
complete document type. A reference DTD provides the basis
for the design of a "family" of related DTDs, such as
subsets, extensions and variants. XHTML 1.1 [XHTML11] acts as a
reference DTD for the XHTML family of document types.
a DTD whose document model is the proper subset of a
reference document type, whose conforming document
instances are still valid according to the reference DTD. A
subset may place tighter restrictions on the markup than
the reference, remove elements or attributes, or both.
a DTD whose document model extends a reference document
type (usually by the addition of element types or
attributes), but generally makes no profound changes to the
reference document model other than required to add the
extension's semantic components. An extension can also be
considered a proper superset if the reference document type
is a proper subset of the extension.
a DTD whose document model alters (through subsetting,
extension, and/or substitution) the basic data model of a
reference document type. It is often difficult to transform
without loss between instances conforming to a variant DTD
and the reference DTD. XHTML Family Conforming Document
Types are not permitted to be "variant DTDs" of the XHTML
a portion of a DTD used as a component either for the
creation of a compound or variant document type, or for
validation of a document fragment. Neither SGML nor XML
current have standardized methods for such partial
the declared markup structure allowed within instances of
an element type. XML 1.0 differentiates two types: elements
containing only element content (no character data) and
mixed content (elements that may contain character data
optionally interspersed with child elements). The latter
are characterized by a content specification beginning with
the "#PCDATA" string (denoting character data).
minimal content model
Some XHTML modules define minimal content models for their
elements. When these modules are used in an XHTML Family
DTD, their content models cannot be altered except that
they may be extended beyond that of the minimal content
a unit of document type specification corresponding to a
distinct type of content, corresponding to a markup
construct reflecting this distinct type.
the definition of an element that is a container for a
distinct semantic class of document content.
an instance of an element type.
the name identifying the element type of an element. Also,
element type name.
descriptive markup delimiting the start and end (including
its generic identifier and any attributes) of an element.
a syntactical construct within a DTD declaring an entity or
defining a markup structure. Within XML DTDs, there are
four specific types: entity declaration defines the binding
between a mnemonic symbol and its replacement content.
element declaration constrains which element types may
occur as descendants within an element. See also content
model. attribute definition list declaration defines the
set of attributes for a given element type, and may also
establish type constraints and default values. notation
declaration defines the binding between a notation name and
an external identifier referencing the format of an
an entity is a logical or physical storage unit containing
document content. Entities may be composed of parse-able
XML markup or character data, or unparsed (ie., non-XML,
possibly non-textual) content. Entity content may be either
defined entirely within the document entity ("internal
entities") or external to the document entity ("external
entities"). In parsed entities, the replacement text may
include references to other entities.
a mnemonic or numeric string used as a reference to the
content of a declared entity (eg., "&" for "&",
"<" for "<", "©" for "©".)
to replace an entity reference with an instance of its
entity an entity whose scope of use is within the document
prolog (ie., the external subset/DTD or internal subset).
Parameter entities are disallowed within the document
an abstract unit within a document model expressed as a DTD
fragment, used to consolidate markup declarations to
increase the flexibility, modifiability, reuse and
understanding of specific logical or semantic structures.
an implementation of a modularization model; the process of
composing or de-composing a DTD by dividing its markup
declarations into units or groups to support specific
goals. Modules may or may not exist as separate file
entities (ie., the physical and logical structures of a DTD
may mirror each other, but there is no such requirement).
the abstract design of the document type definition (DTD)
in support of the modularization goals, such as reuse,
extensibility, expressiveness, ease of documentation, code
size, consistency and intuitiveness of use. It is important
to note that a modularization model is only orthogonally
related to the document model it describes, so that two
very different modularization models may describe the same
a generally short file used to declare and instantiate the
modules of a DTD. A good rule of thumb is that a DTD driver
contains no markup declarations that comprise any part of
the document model itself.
parent document type
A parent document type of a compound document is the
document type of the root element.
A compound document is a document that uses more than one
XML Namespace. Compound documents may be defined as
documents that contain elements or attributes from multiple
A module is a collection of elements, attributes, values
for attributes, content models, or any combination of