I frequently use a structure consisting of
\chapter, \section, \subsection, and
The only real constraint, I think, is that without special adjustments you can only number the structure above the point where you "break the hierarchy".
But then applications where you number down to 5 levels (or 6 if you add
\subparagraph) are, I think, very specialised and somewhat unusual.
I apologise: this is a really bad answer, in that I am simply transferring my laziness to others. As noted by @DavidCarlisle and @FrankMittelbach, what I'm doing here is opting for convenience in place of semantic rigour.
I have always, really, been an advocate of the theory "Don't tell lies to your computer, because ten years later, after you've forgotten, it will tell those same lies back to you", but Frank and David have, rightly, called me out for having my fingers in the till.