when should that be used in defining clauses. Is it only used to give more information about a person or something or can it also be used to give more information about a place. Is it informal or formal English?
"That" introduces restrictive relative clauses that contain no relative pronoun corresponding to the phrase modified by the relative clause. It doesn't matter what that omitted relative pronoun would have been or what it would have referred to. I don't think it has anything to do with formality.