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       SECTION 10.6                                                        Logical Structure



10.6.4 Structure Attributes

       An application or plug-in extension that processes logical structure can attach ad-
       ditional information, called attributes, to any structure element. The attribute in-
       formation is held in one or more attribute objects associated with the structure
       element. An attribute object is a dictionary or stream that includes an O entry
       (see Table 10.14) identifying the application or plug-in that owns the attribute
       information. Other entries represent the attributes: the keys are attribute names,
       and values are the corresponding attribute values. To facilitate the interchange of
       content among PDF applications, Adobe has defined a set of standard structure
       attributes identified by specific standard owners; see Section 10.7.4, “Standard
       Structure Attributes.” In addition, PDF 1.6 introduces a use of attributes to repre-
       sent user properties (see “User Properties” on page 876).

                  TABLE 10.14 Entry common to all attribute object dictionaries
KEY   TYPE       VALUE

O     name       (Required) The name of the application or plug-in extension owning the attribute data.
                 The name must conform to the guidelines described in Appendix E.

       Any application can attach attributes to any structure element, even one created
       by another application. Multiple applications can attach attributes to the same
       structure element. The A entry in the structure element dictionary (see Table
       10.10 on page 858) can hold either a single attribute object or an array of such
       objects, together with revision numbers for coordinating attributes created by dif-
       ferent owners (see “Attribute Revision Numbers” on page 874). An application
       creating or destroying the second attribute object for a structure element is re-
       sponsible for converting the value of the A entry from a single object to an array
       or vice versa, as well as for maintaining the integrity of the revision numbers. No
       inherent order is defined for the attribute objects in an A array, but it is consid-
       ered good practice to add new objects at the end of the array so that the first ar-
       ray element is the one belonging to the application that originally created the
       structure element.

       Attribute Classes

       If many structure elements share the same set of attribute values, they can be
       defined as an attribute class sharing the identical attribute object. Structure
       elements refer to the class by name. The association between class names and

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