Reg-T (equity) margin versus performance bond (futures)
In equities markets, margin generally refers to Reg-T margin. Reg-T margin allows the broker to lend up to 50% of the purchase price of the equities to the client. The transaction must be completed in a margin account, and the broker can charge interest on the amount borrowed.
In futures markets, futures contracts are assigned a margin rate or performance bond, (usually 3-12% of the contract's notional value) that customers must have in their trading account with their broker if they intend to hold the futures position overnight.
Initial margin is the amount of funds necessary to open a futures position. Initial margin is 10% higher than maintenance margin.
Maintenance margin is the minimum amount that must be kept in the trading to maintain the futures position.
For example, if a customer has a futures position in their trading account at the end of the trading day - generally 4pm US central time - the broker requires that the customer have a cash balance equal to or greater than the maintenance margin of the position, otherwise the account will be on margin call.
Unlike equities, where Reg-T margin changes based on the underlying value of the equities, margins on futures products are controlled and published by the exchange and generally do not change very frequently.