31 CFR § 50.30 - General participation requirements.
(a) Insurers. As defined in § 50.4(o), all state residual market insurance entities and state workers' compensation funds are insurers under the Program even if such entities do not receive direct earned premiums.
(b) Mandatory participation. State residual market insurance entities and State workers' compensation funds are mandatory participants in the Program subject to the rules issued in this Subpart.
(c) Identification. Treasury maintains a list of state residual market insurance entities and state workers' compensation funds at https://home.treasury.gov/policy-issues/financial-markets-financial-institutions-and-fiscal-service/federal-insurance-office/terrorism-risk-insurance-program. Procedures for providing comments and updates to that list are posted with the list.