Adobe provides slots for: 1) oldstyle figures (from uniF730 to uniF739); 2) small caps (from uniF761 to uniF77A); 3) ligatures (from uniFB00 to uniFB06).
However in most fonts those glyphs are freely inserted in PUA. Not all, for in some cases ligatures are just in the predefined slots.
So: a) is it correct or by now obsolete to place those glyphs in the slots provided by Adobe, which it seems to me for some time advises against their use, and put them in PUA instead?
b) why instead in the case of ligatures do some fonts use the predefined slots, other ones put them in the PUA, and what is the best solution?