Section text (sec) can be a simple text provided by the user, or it can be more structured. It can be a list of subsections and may have an introductory clause (0.sec) and may have an exit clause (00.sec). The list of sections may use any of the HTML numbering formats - 1, I, i, A, a, bullet, etc.
This set of defaults avoids the need for cluttering a list with a separate Z/ol/0 for each section that has only a simple sec text. It also forces you to put the reference to any Z/… at the bottom.