This subthread is confusing clearinghouses, clearing brokers, and execution brokers.
Citadel is an execution broker. Their job is to match buyers and sellers.
Apex (in these conversations) is a clearing broker. Robinhood is its own clearing broker. Clearing brokers are responsible for ensuring that money and stock actually changes hands (this takes 2 days, but all of these firms work together to create the illusion, using credit, that it's instant; those credit arrangements are why brokerages post collateral).
Clearing brokers are members of clearinghouses. The relevant one here is (I think) NSCC, which is owned by DTCC. Policy set at DTCC determines how much collateral needs to be posted to cover any particular set of trades.
DTCC drastically ratcheted up the amount of collateral required to cover trades in meme stocks, which had the effect of 10x'ing the amount of cash Robinhood was required to post to insure that it would not go out of business before its current set of in-flight trades cleared. It made the same requirement of Apex, which passed restrictions down to its customers. These companies are contractually required to make good on collateral requirements, so there isn't much choice involved.
Citadel is an execution broker. Their job is to match buyers and sellers.
Apex (in these conversations) is a clearing broker. Robinhood is its own clearing broker. Clearing brokers are responsible for ensuring that money and stock actually changes hands (this takes 2 days, but all of these firms work together to create the illusion, using credit, that it's instant; those credit arrangements are why brokerages post collateral).
Clearing brokers are members of clearinghouses. The relevant one here is (I think) NSCC, which is owned by DTCC. Policy set at DTCC determines how much collateral needs to be posted to cover any particular set of trades.
DTCC drastically ratcheted up the amount of collateral required to cover trades in meme stocks, which had the effect of 10x'ing the amount of cash Robinhood was required to post to insure that it would not go out of business before its current set of in-flight trades cleared. It made the same requirement of Apex, which passed restrictions down to its customers. These companies are contractually required to make good on collateral requirements, so there isn't much choice involved.