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             CHAPTER 10                                                        Document Interchange



KEY               TYPE                 VALUE

Lang              text string          (Optional; PDF 1.4) A language identifier specifying the natural language
                                       for all text in the structure element except where overridden by language
                                       specifications for nested structure elements or marked content (see Sec-
                                       tion 10.8.1, “Natural Language Specification”). If this entry is absent, the
                                       language (if any) specified in the document catalog applies.

Alt               text string          (Optional) An alternate description of the structure element and its
                                       children in human-readable form, which is useful when extracting the
                                       document’s contents in support of accessibility to users with disabilities
                                       or for other purposes (see Section 10.8.2, “Alternate Descriptions”).

E                 text string          (Optional; PDF 1.5) The expanded form of an abbreviation.

ActualText        text string          (Optional; PDF 1.4) Text that is an exact replacement for the structure
                                       element and its children. This replacement text (which should apply to
                                       as small a piece of content as possible) is useful when extracting the doc-
                                       ument’s contents in support of accessibility to users with disabilities or
                                       for other purposes (see Section 10.8.3, “Replacement Text”).


 10.6.2 Structure Types

             Every structure element has a structure type, a name object that identifies the
             nature of the structure element and its role within the document (such as a chap-
             ter, paragraph, or footnote). To facilitate the interchange of content among PDF
             applications, Adobe has defined a set of standard structure types; see Section
             10.7.3, “Standard Structure Types.” Applications are not required to adopt them,
             however, and may use any names for their structure types.

             Where names other than the standard ones are used, a role map may be provided
             in the structure tree root, mapping the structure types used in the document to
             their nearest equivalents in the standard set. For example, a structure type named
             Section used in the document might be mapped to the standard type Sect. The
             equivalence need not be exact; the role map merely indicates an approximate
             analogy between types, allowing applications other than the one creating a docu-
             ment to handle its nonstandard structure elements in a reasonable way.

             Note: The same structure type may occur as both a key and a value in the role map,
             and circular chains of association are explicitly permitted. A single role map can
             thus define a bidirectional mapping. An application using the role map should fol-

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