> custodian, trading exchange, market maker, and Kyc/Aml
Custodians are people who hold on to funds and securities. If a mutual fund buys a share of Apple, they don't hold the share themselves, it's held by a custodian bank like BNY.
> trading exchange
NYSE/NASDAQ/CBOE/etc are examples of exchanges, where they match up buy orders and sell orders, and nothing more. Dark pools are examples of exchanges, as they are a private place that match up buy/sell orders.
> market maker
Market makers are, as their name suggests, institutions that exist to make markets. What this often means is that they have the obligation to maintain both buy and sell orders on specific securities they signed up to make markets for at all times. Examples include Jane Street, Citadel, etc.
> kyc/aml
GP probably mistyped; kyc/aml are regulations that must be followed by financial firms interacting with their customers. Applies to basically everybody here.
A few more:
Clearinghouses: when you trade a security on an exchange, you mere commit to buying x for $y at $settlement date, typically 2 business days after the trade happens. This settlement happens at a clearinghouse, which guarantees both sides of the trade.
The objection from most regulators is that a crypto exchange typically acts as all of the above simultaneously. They are custodians, in that they hold the crypto, they typically make markets, they are obviously an exchange, they provide clearing and settlement services, all at once. Regulators don't like this, since there are conflicts of interest inherent in the roles and they should be separated out into different institutions.
