Mix of paragraphs and styles/levels of headings (H1, H2, ..) — (HTML, most word processors)
Explicit section levels, with level-designated headings — (sec1 contains head1, paras; sec2 contains head2, paras; sec3 contains head3, paras)
Nested explicit section levels, with level-designated headings — (sec1 contains head1, paras, & sec2; sec2 contains head2, paras, & sec3 ...)
Recursive sections with undesignated headings — (sec contains head, paras, & sec; sec contains head, paras, & sec ...)
(IMO: the only wrong/bad choice on section model is to allow more than one of these in a document set.)
Use the section model of your source vocabulary — JATS and JATS-compatible models use recursive sections