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 Modularization of XHTML specification[XML] is highly recommended.

document type: 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)"
document model: 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.
markup model: 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.
reference DTD: 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.
subset DTD: 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.
extension DTD: 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.
variant DTD: 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.
fragment DTD: 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. SGML nor XML current have standardized methods for such partial validation.
content model: 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).
abstract module: a unit of document type specification corresponding to a distinct type of content, corresponding to a markup construct reflecting this distinct type.
element type: the definition of an element, that is, a container for a distinct semantic class of document content.
element: an instance of an element type.
generic identifier: the name identifying the element type of an element. Also, element type name.
tag: descriptive markup delimiting the start and end (including its generic identifier and any attributes) of an element.
markup declaration: 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 unparsed entity
entity: 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.
instantiate: to replace an entity reference with an instance of its declared content.
parameter: 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 instance.
module: 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.
modularization: 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).
modularization model: 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 document type.
driver: 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.
compound document: 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 document types.
module: A module is a collection of elements or attributes.