Optional regulation of standard contract terms gives rise to several functional peculiarities and to a number of difficult systematic questions. More specifically, the present chapter deals with the draft rules on standard contract terms in the proposed Common European Sales Law (CESL). Rather than commenting on these rules in substantially, however, its focus is precisely on these functional questions. We proceed in three steps, starting with an overview of the existing European rules on standard contract terms, including the draft provisions of the CESL. In a second step, we examine whether the CESL rules themselves could potentially become subject of control under (national) standard contract terms legislation, given that these rules are provided in a standard format, and that they are adopted by the contracting parties’ opt-in basis. Thirdly and finally, we briefly analyse the mode of function of the CESL’s own rules on standard contract terms. These three steps will show that according to the European legislator’s design of the opt-in mechanism, two different optional contract law regimes operate within the very same national legal system. This two-foldness implies ambiguities, because the control of standard terms strongly interacts with substantial rules of contract law. This interaction is a necessary, unavoidable consequence of the embeddedness of the optional regime in national contract law.