1) According to your reply: "that" as a relative pronoun cannot be used in "non-identifying" clauses. Does it mean that "which" can be used in all cases (except for persons)? Or are there any cases where only "that" can be used?
2) Is this an "identifying" or "non-identifying" clause:
We need to cover all the facts relating to the accounting unit, which are of significance with regard to the audited financial statements.
1- In non-identiying clauses, we use which/who.
2- I think this should be identifying which facts need to be covered, so there shouldn't be the comma.
I think you don't need all facts ralating to the accounting unit, but a subset of those facts, namely to ones related to the audit. If you leave the comma in, you'll get a lot of unnecessary information, I think. As a test, think whether you can remove the relative clause from the sentence without changing the basic meaning. If you can, then it's just extra information and non-identifying. If you change the meaning significantly or have an incomplete sentence, then it's identifying. It could be non-identifying, but that would imply that the accounting unit only has facts about the audit, which doesn't seem that likely to me.