16.5.7 Specifying full ancestry for nested sections
When you have nested DocBook sections you must specify parentage starting with $top for every section title. For example:
Heading2 = $top section section
Heading3 = $top section section section
Heading4 = $top section section section section
Otherwise, the higher levels would also match the rule for the lower levels; so, for example, the following settings:
would allow another Heading1 section to follow a Heading2 section without closing the lower-level Heading2 section. The starting $top prevents this.
In addition, you would need to specify:
so that each heading starts a new section when it occurs at the same level as the preceding section. Otherwise a second Heading2 section would be valid inside the first Heading2 section, and would not close that section and start a new section of its own at the same level.
§16.5.5 Specifying alternate ancestries for the same element
§16.5.6 Specifying first-child status for nested elements
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