This specification defines an abstract data set called

　　the XML Information Set

　　(Infoset).

　　Its purpose is to provide a consistent set of definitions for use

　　in other specifications that need to refer to the information in a well-formed

　　XML document [XML].

　　It does not attempt to be exhaustive; the primary criterion for inclusion

　　of an information item or property has been that of expected usefulness

　　in future specifications. Nor does it constitute a minimum set of

　　information that must be returned by an XML processor.

　　An XML document has an information set if it is well-formed and

　　satisfies the namespace constraints described

　　below.

　　There is no requirement

　　for an XML document to be valid in order to have an information set.

　　Information sets may be created by methods (not described in this

　　specification) other than parsing an XML document.

　　See Synthetic Infosets below.

　　An XML document's information set consists of a number of

　　information items;

　　the information set for any well-formed XML document

　　will contain at least a

　　document information item

　　and several others.

　　An information item is an abstract description of some part of an XML

　　document: each information item has a set of associated named

　　properties. In this specification, the

　　property names are shown in square brackets, [thus].

　　The types of information item are listed in

　　section 2.

　　The XML

　　Information Set does not require or favor a specific interface or class of

　　interfaces. This specification presents the information set as a modified

　　tree for the sake of clarity and simplicity, but there is no requirement that

　　the XML Information Set be made available through a tree structure; other

　　types of interfaces, including (but not limited to) event-based and query-based

　　interfaces, are also capable of providing information conforming to the XML

　　Information Set.

　　The terms "information set" and "information

　　item" are similar in meaning to the generic terms "tree" and "node", as they

　　are used in computing. However, the former terms are used in this specification

　　to reduce possible confusion with other specific data models. Information

　　items do not map one-to-one with the nodes of the DOM or the "tree"

　　and "nodes" of the XPath data model.

　　In this specification, the words "must",

　　"should", and "may" assume the meanings specified in

　　[RFC2119], except that the words do not appear in

　　uppercase.

　　XML documents that do not conform to

　　[Namespaces],

　　though technically well-formed,

　　are not considered to have meaningful information sets.

　　That is, this specification does not define an information

　　set for documents that have element or attribute names containing colons that

　　are used in other ways than as prescribed by

　　[Namespaces].

　　Furthermore, this specification does not define an information set for

　　documents which use relative URI references in namespace declarations.

　　This is in accordance with the decision of the W3C XML Plenary Interest

　　Group described in [Relative Namespace URI References].

　　The value of a [namespace name] property is the normalized value of

　　the corresponding namespace attribute; no additional URI escaping is

　　applied to it by the processor.

　　An information set describes its XML document with entity

　　references already expanded, that is, represented by the information

　　items corresponding to their replacement text. However, there are

　　various circumstances in which a processor may not perform this

　　expansion. An entity may not be declared, or may not be retrievable.

　　A non-validating processor may choose not to read all declarations,

　　and even if it does, may not expand all external entities. In these

　　cases an

　　unexpanded entity reference

　　information item is used to represent the entity reference.

　　The values of all properties in the Infoset

　　take account of the end-of-line normalization described in

　　[XML], 2.11 "End-of-Line Handling".

　　Several information items have a [base URI] or [declaration base URI] property.

　　These are computed according to

　　[XML Base].

　　Note that retrieval of a resource may involve redirection

　　at the parser level (for example, in an entity resolver) or below;

　　in this case the base URI is the final URI used to retrieve the resource

　　after all redirection.

　　The value of these properties does not reflect any URI escaping that

　　may be required for retrieval of the resource, but it may include

　　escaped characters if these were specified in the document, or returned

　　by a server in the case of redirection.

　　In some cases (such as a document read from a string or a pipe) the

　　rules in

　　[XML Base]

　　may result in a base URI being application

　　dependent. In these cases this specification does not define

　　the value of the [base URI] or [declaration base URI] property.

　　When resolving relative URIs the [base URI] property should be used in

　　preference to the values of xml:base attributes; they may be inconsistent

　　in the case of Synthetic Infosets.

　　Some properties may sometimes have the value

　　unknown or

　　no value,

　　and it is said that a property value is unknown or that a property

　　has no value respectively.

　　These values are distinct from each other and from all other values.

　　In particular they are distinct from the empty string, the empty set,

　　and the empty list, each of which simply has no members.

　　This specification does not use the term null since in some

　　communities it has particular connotations which may not match those

　　intended here.

　　This specification describes the information set resulting from parsing

　　an XML document. Information sets may be constructed by other means,

　　for example by use of an API such as the DOM or by transforming an

　　existing information set.

　　An information set corresponding to a real document will necessarily

　　be consistent in various ways; for example the [in-scope namespaces]

　　property of an element will be consistent with the [namespace

　　attributes] properties of the element and its ancestors. This may not

　　be true of an information set constructed by other means; in such a case

　　there will be no XML document corresponding to the information set,

　　and to serialize it will require resolution of the inconsistencies

　　(for example, by outputting namespace declarations that correspond to

　　the namespaces in scope).