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                                        904
CHAPTER 10                                                     Document Interchange



Note: The association of headers with rows and columns of data is typically deter-
mined heuristically by applications. Such heuristics may fail for complex tables; the
standard attributes for tables shown in Table 10.36 can be used to make the associa-
tion explicit.

Usage Guidelines for Block-Level Structure

Because different consumer applications use PDF’s logical structure facilities in
different ways, Tagged PDF does not enforce any strict rules regarding the order
and nesting of elements using the standard structure types. Furthermore, each
export format has its own conventions for logical structure. However, adhering to
certain general guidelines helps to achieve the most consistent and predictable in-
terpretation among different Tagged PDF consumers.

As described under “Grouping Elements” on page 899, a Tagged PDF document
can have one or more levels of grouping elements, such as Document, Part, Art
(Article), Sect (Section), and Div (Division). The descendants of these are BLSEs,
such as H (Heading), P (Paragraph), and L (List), that hold the actual content.
Their descendants, in turn, are either content items or ILSEs that further describe
the content.

Note: As noted earlier, elements with structure types that would ordinarily be treat-
ed as ILSEs can have a Placement attribute (see “General Layout Attributes” on
page 917) that causes them to be treated as BLSEs instead. Such elements may be in-
cluded as BLSEs in the same manner as headings and paragraphs.

The block-level structure can follow one of two principal paradigms:

• Strongly structured. The grouping elements nest to as many levels as necessary
  to reflect the organization of the material into articles, sections, subsections,
  and so on. At each level, the children of the grouping element consist of a head-
  ing (H), one or more paragraphs (P) for content at that level, and perhaps one or
  more additional grouping elements for nested subsections.
• Weakly structured. The document is relatively flat, having perhaps only one or
  two levels of grouping elements, with all the headings, paragraphs, and other
  BLSEs as their immediate children. In this case, the organization of the material
  is not reflected in the logical structure; however, it can be expressed by the use
  of headings with specific levels (H1–H6).

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