Cal. Civ. Proc. Code § 2033.010

Any party may obtain discovery within the scope delimited by Chapters 2 (commencing with Section 2017.010) and 3 (commencing with Section 2017.710), and subject to the restrictions set forth in Chapter 5 (commencing with Section 2019.010), by a written request that any other party to the action admit the genuineness of specified documents, or the truth of specified matters of fact, opinion relating to fact, or application of law to fact. A request for admission may relate to a matter that is in controversy between the parties.