The document discusses the use of relative clauses with who, which, and that. It provides examples of defining relative clauses that identify a specific person or thing. It explains that who is used to refer to people, that or which can refer to things, and that which is more formal. Non-defining relative clauses add extra information and are set off by commas. Who, whom, or which are used in non-defining clauses, depending on if they refer to a person or thing and their function in the clause.