FinCEN defines exchange houses as financial institutions licensed to deal in foreign exchange and transmit funds on behalf of individuals and companies. The designations described on this site concern Iran-based exchange houses that, according to Treasury, hold and move foreign currency for sanctioned Iranian banks.
Treasury describes those houses as often family-run general partnerships whose partners are ultimately liable for the funds entrusted to them by the banks. Each runs its own network of front companies abroad, with foreign bank accounts used in several currencies.