Symbols which are not defined anywhere mean absolutely nothing. Therefore, I think we should maintain a list of conventional symbols used on ELL.
*: Used in front of an example word or phrase to indicate that the example is ungrammatical (e.g. *Me enjoy eating cookies.)
?: Used similarly to *, but indicates a word or phrase that is potentially marginal or awkward, rather than fully ungrammatical (e.g. ?unbelievableness)
??: The same as ?, but indicating stronger doubt.
#: Used to indicate semantic incoherence (e.g. #Colorless green ideas sleep furiously.)
/x/ indicates a phonemic transcription, where x is one or more IPA symbols.
[x] indicates a phonetic transcription, where x is one or more IPA symbols.
⟨x⟩ indicates a grapheme (how something looks in an ordinary writing system).
This might be downvoted by those who say abbreviations aren't "symbols", and/or those who disapprove of these particular abbreviations being used at all, but I think here is a perfectly good place to identify:
These forms are used by some dictionaries, and I've no objection to them being used on ELL. But I'm not bothered if people with edit privileges want to change them to the full forms.
The following symbols are used in The Cambridge Grammar of the English Language, p. xii:
* ungrammatical *This books is mine.
# semantically or pragmatically anomalous #We frightened the cheese.
% grammatical in some dialect(s) only %He hadn't many friends.
? of questionable grammaticality ?Sue he gave the key.
! non-standard !I can't hardly hear.
We can use these symbols similarly, if we like.
If you do use these symbols in an answer, I suggest adding a footnote explaining how you're using them. That way, anyone can understand your answer, even if they're not familiar with the symbols.