No rules really! Broadly speaking rare items are no longer made or are very limited numbers in current season.
I meant as in sought after and hard to get, unusual, perhaps a short production run or only a few ever made. Speciality leathers, different colours/finishes. Collectors items.
Personally I like bags with tooling or applique and these are almost always rather hard to find as they're not everyones cup of tea so fewer are produced.