Module: Headings
Module: Headings
<p>
In PDF, a <dfn>heading</dfn> is “[a] label for a subdivision of a document’s content” (PDF 1.7, §10.7.3 “Standard Structure Types”). A compliant document must use heading tags if its content contains headings.
</p>
<p>
PDF generally, and PDF/UA in particular, provide for two categories of headings – one unnumbered heading tag for use within structured sections and six or more numbered heading tags for use outside of structured sections.
</p>
<h2>Section tags</h2>
<p>
PDF provides a number of tags to be used for structuring documents (PDF 1.7, §10.7.3, “Grouping Elements”). Some structural tags may have a semantic meaning unto themselves; others have no intrinsic semantics and are merely used to group elements.
</p>
<p>
For PDF/UA, the following structural tags (listed alphabetically) are always considered relevant to the use of headings:
</p>
<ul>
<li><code>Art</code></li>
<li><code>BlockQuote</code></li>
<li><code>Caption</code></li>
<li><code>Div</code></li>
<li><code>Document</code></li>
<li><code>Index</code></li>
<li><code>NonStruct</code></li>
<li><code>Part</code></li>
<li><code>Sect</code></li>
<li><code>TOC</code></li>
</ul>
<p>
If none of these tags is appropriate for document content, authors may define their own structural tags. If such author-defined tags have grouping functions similar to the listed tags, they must be treated as though they form part of that list.
</p>
<h2>Numbered headings</h2>
<p>
For documents, or portions of documents, that are not divided into sections or logical units, heading tags <code>H1</code> through <code>H6</code> must be used.
</p>
<ul>
<li><code>H1</code> is the highest level. <code>H6</code> is the lowest.</li>
<li>If any heading tags are used, <code>H1</code> must be the first.</li>
<li>A document may use more than one instance of any specific tag level. For example, a tag level may be repeated if document content requires it (e.g., <code>H1 H2 H3 H3</code> is a valid sequence if the content has one top-level heading, one second-level heading, and two consecutive third-level headings).</li>
<li>A document may not skip a level when decrementing its tag sequence: <code>H1 H2 H3</code> is permissible, while <code>H1 H3</code> is not.</li>
<li>A document may increment its heading sequence without restarting at <code>H1</code> if document content requires it: <code>H1 H2 H3 H4 H3 H4 H3 H4 H2 H3</code> is a permissible sequence.</li>
</ul>
<h3>Additional heading levels</h3>
<p>
If a PDF that contains the tags <code>H1</code>, <code>H2</code>, <code>H3</code>, <code>H4</code>, <code>H5</code>, and <code>H6</code> (in any permissible sequence) requires more headings, authors may define and use tags numbered from <code>H7</code> upward (without limit). Tags <code>H7</code> through <code>H<var>n</var></code> must comply with all the usage requirements listed above for <code>H1</code> through <code>H6</code>. These user-defined heading tags must use the letter <code>H</code> plus Arabic numerals, must not use any other numbering system (e.g., Roman numerals), and must not use numerical separators (e.g., <code>H1247</code> is permissible while <code>H1,247</code> and <code>H1 247</code> are not).
</p>
<h3>Interpretation of synonymous tags</h3>
<p>
A conforming user agent must interpret <code>Heading<var>n</var></code> or <code>Header<var>n</var></code> as being equivalent to <code>H<var>n</var></code>, for 1≤<var>n</var>≤6. Numerals in these tag strings may be preceded by whitespace. For example, <code>Heading2</code> and <code>Header 2</code> are equivalent in all respects to <code>H2</code>.
</p>
<p>
A conforming user agent may consider tags of the form <code>Heading<var>n</var></code> or <code>Header<var>n</var></code>, where <var>n</var>>6, as equivalent to <code>H<var>n</var></code> with a corresponding numeric value. For example, <var>Header 10</var> may be considered equivalent to <code>H10</code>.
</p>
<h2>Unnumbered headings</h2>
<p>
Documents, or portions of documents, that use section tags for structuring content shall not use numbered headings. Only the generic heading <code>H</code> may be used (PDF 1.7, §10.7.3, “Paragraphlike Elements”).
</p>
<p>
Each instance of <code>H</code> shall have one section-tag parent. To indicate successive levels of headings, authors must nest other section tags; each of those nested section tags may contain at most one <code>H</code>.
</p>
<p>
For example, consider the numbered-tag sequence <code>H1 H2 H3</code>. Using section tags, some legal tag sequences with meanings equivalent to that sequence are as follows:
</p>
<ol>
<li>
<ul>
<li><code>Sect</code>
<ul><li><code>H</code>
<ul><li><code>Sect</code>
<ul><li><code>H</code>
<ul><li><code>Sect</code>
<ul><li><code>H</code>
</li></ul></li></ul></li></ul></li></ul></li></ul></li>
</ul>
</li>
<li>
<ul>
<li><code>Part</code>
<ul><li><code>H</code>
<ul><li><code>Sect</code>
<ul><li><code>H</code>
<ul><li><code>Art</code>
<ul><li><code>H</code>
</li></ul></li></ul></li></ul></li></ul></li></ul></li>
</ul>
</li>
<li>
<ul>
<li><code>Sect</code>
<ul><li><code>H</code>
<ul><li><code>Art</code>
<ul><li><code>H</code>
<ul><li><code>BlockQuote</code>
<ul><li><code>H</code>
</li></ul></li></ul></li></ul></li></ul></li></ul></li>
</ul>
</li>
</ol>
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