One might not be, unless you’re asking for variable behavior depending on which option one selects.
The custom setting will let user set any arbitrary interval that they prefer, whether ot not you prefer to.
Maybe that’s too flexible, but it was there before the thinning-with-age retention feature was added in.
As mentioned (and linked), weeks start on various days, which might be Saturday, Sunday, or Monday.
It gets much worse the further one gets into calendar. If you would like see “convoluted”, look this over:
and after that, look left at the other topics on the bar. Looks hugely more complex than using intervals.
Maybe it simplifies to something easier than I think, but at least the feature developer didn’t go there…
Some of the original discussions are below. Things such as “week” variations came up several times.
Staggered versioning / thinning out versions
Retention Policy like Time Machine [$50] #2084
New retention policy deletes old backups in a smart way
While I agree something “should” be done to reduce accidents, I’m not sure that “calendar” is the way.
With a very large backlog of issues and requests, development must keep development effort in mind.