To specify contents levels, assign level numbers to heading formats that start topics, and also to formats to which you have assigned the Contents property; see §16.4.2 Including contents entries in HTML-based Help. Level 1 is the top level; each higher level number represents another level of indentation in the TOC.
[HelpContentsLevels]
; FM paragraph format name = TOC level
PrefTitle = 1
ChapTitle = 1
AppxTitle = 1
Head1 = 2
Head2 = 3
Head3 = 4
AppxHead1 = 2
AppxHead2 = 3
You can specify any paragraph format, and all text in that format will appear in the table of contents. You are not restricted to paragraph formats that are mapped to HTML Hn tags (see §30.2.1 Assigning HTML tags and attributes to paragraph formats).