Because retail spot forex in the US is an off-exchange contract with the broker as counterparty, the law requires that counterparty to be registered and capitalised. RFEDs and futures commission merchants dealing in retail forex must hold substantial adjusted net capital and file regularly, which is why the field is small.
The category is worth knowing because it sets the terms of the relationship: your trade is with the dealer, as described in counterparty-risk, not on an exchange, and US rules require positions to be managed under the fifo-rule with no offsetting long and short in the same pair.
Solicitation of US residents by unregistered foreign brokers is prohibited, which is the reason many international firms decline US sign-ups outright.
Example: a US resident comparing brokers can check registration status on the NFA's public database before funding, in the same way a UK client checks the fca register.
Related: us-leverage-limits, nfa-compliance-rule-2-43, counterparty-risk, otc-market