A more thorough explanation:

A margin list is a list created by the Federal Reserve Board that limits the amount of money a bank can borrow using its own stock as collateral. The limit is usually a percentage of the stock's market value, such as 50%. If a bank is not on the margin list, there is no limit to the amount of money it can borrow using its stock as collateral.

Bank A is on the margin list with a limit of 50%. This means that if Bank A's stock is worth $100, the maximum amount it can borrow using its stock as collateral is $50. If Bank A's stock is worth $200, the maximum amount it can borrow is $100.

Bank B is not on the margin list. This means that if Bank B's stock is worth $100, it can borrow any amount using its stock as collateral. If Bank B's stock is worth $200, it can borrow any amount up to $200.

These examples illustrate how the margin list works. It is a way for the Federal Reserve Board to regulate the amount of money banks can borrow using their own stock as collateral. Banks on the margin list have a limit, while banks not on the list have no limit.