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                                                  619
         SECTION 8.4                                                                  Anno ta tions



KEY          TYPE          VALUE

Subj         text string   (Optional; PDF 1.5) Text representing a short description of the subject being
                           addressed by the annotation.

RT           name          (Optional; meaningful only if IRT is present; PDF 1.6) A name specifying the rela-
                           tionship (the “reply type”) between this annotation and one specified by IRT. Val-
                           id values are:
                              R         The annotation is considered a reply to the annotation specified by
                                        IRT. Viewer applications should not display replies to an annotation
                                        individually but together in the form of threaded comments.
                              Group     The annotation is grouped with the annotation specified by IRT; see
                                        discussion below.
                           Default value: R.

IT           name          (Optional; PDF 1.6) A name describing the intent of the markup annotation. In-
                           tents allow viewer applications to distinguish between different uses and behav-
                           iors of a single markup annotation type. If this entry is not present or its value is
                           the same as the annotation type, the annotation has no explicit intent and should
                           behave in a generic manner in a viewer application.
                           Free text annotations (Table 8.25), line annotations (Table 8.26), polygon anno-
                           tations (Table 8.29), and (in PDF 1.7) polyline annotations (Table 8.29) have de-
                           fined intents, whose values are enumerated in the corresponding tables.

ExData       dictionary    (Optional; PDF 1.7) An external data dictionary specifying data to be associated
                           with the annotation. This dictionary contains the following entries:
                              Type (optional): If present, must be ExData.
                              Subtype (required): a name specifying the type of data that the markup anno-
                                   tation is associated with. In PDF 1.7, the only defined value is
                                   Markup3D.
                           For each value of Subtype, other entries are defined. Table 9.48 on page 835
                           lists the values that correspond to a subtype of Markup3D. (See also
                           implementation note 96 in Appendix H.)

         In PDF 1.6, a set of annotations can be grouped so that they function as a single
         unit when a user interacts with them. The group consists of a primary annotation,
         which must not have an IRT entry, and one or more subordinate annotations,
         which must have an IRT entry that refers to the primary annotation and an RT en-
         try whose value is Group.

         Some entries in the primary annotation are treated as “group attributes” that
         should apply to the group as a whole; the corresponding entries in the subordi-

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