I too find the whole USFM documentation thing confusing. Most of the time the online website documentation seems too simplistic and doesn’t answer the question I go it looking for the answer to.
So, I’m not sure the following is true because I don’t fully understand what USFM is and isn’t trying to define… but, I think that \OccursUnder and the related \Rank are PT specific markers which are used to help PT handle format markers. In other words, (if I’m right) they’re not actually part of the USFM standard which is why they’re not included in the documentation.
Inside of PT, the help article
How does the stylesheet specify where a marker is allowed to occur? might be useful.