Thus in the current official IPA raised symbols may denote
- phonation features, more specifically VOT (ʰ),
- secondary articulations (ʷ, ʲ, ˠ,ˤ), or
- types of plosive release (n, l).
It is worth mentioning that at no time has the IPA used raised symbols to denote quick transitional sounds, although that is what people often wrongly infer as the meaning of this convention. Nor does ə denote a “weak” schwa, as one of my recent correspondents assumed. (In English every ə is weak, anyway, in my terminology.)
All uses of raised alphabetic symbols other than those listed above are ad hoc conventions that must be defined by the author who uses them. (That’s why I defined what I meant by ˀ the other day when I used it to show the Danish stød.)
In LPD I use raised symbols to denote optional sounds that the EFL learner is advised to ignore (“although native speakers sometimes include them” — see the panel on Optional Sounds, p. 567 in the third edition).
I use them in particular for
- the əC alternative to a syllabic consonant, thus hidden ˈhɪd ən
- possible epenthetic plosives, thus emphasis ˈempf əs ɪs;
- possible epenthetic schwa before a liquid, thus fail feɪəl.