I'm not quite sure what you mean here. The relative pronoun 'which' can't be used to refer to people. The relative pronoun 'that' isn't used in non-defining relative clauses, which basically means after a comma. 8)
: What he said shocked me. (In this sentence what carries the meaning the thing that. So the way to test whether to use what or not, is to plave this phrase into the sentence and see if it works. :lol: