Special memorandum account

Special memorandum account (SMA) is a margin credit account used for calculating US Regulation T requirements on brokerage accounts. In addition to Initial Margin and "Maintenance Margin" requirements, the SMA ledger is used to lock in unrealized gains that augment the client's buying power.
According to Regulation T, Section 220.6: 
(b) The SMA may contain the following entries:
Regulation T allows transfers from the SMA to be used as margin for new purchases in their margin account. However, exchange rules do not allow these transfers to be used for maintenance margin calls. The SMA balance represents credits that are used only for meeting margin requirements and are not actual funds that could be withdrawn by the client.
"Buying Power" is always twice the SMA balance.