It’s part of a much messier legal battle playing out between federal regulators and several states over what event contracts actually are. They argue that certain event contracts — especially those tied to sports outcomes — are basically unlicensed gambling dressed up in derivatives language. It shapes what contracts platforms can offer, in which states, to which users, and — now — how those contracts can be marketed. It tries to shore up the regulatory legitimacy of event contracts by insisting they look and feel like financial instruments. The letter went to exchanges offering event contracts broadly, including introducing brokers and futures commission merchants.