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How Does A Special Memorandum Account Work
Here is another explanation of an SMA.  SMA refers to the Special Memorandum Account which represents neither equity nor cash but rather a line of credit created when the market value of securities in a Reg. T margin account increase in value.  For example, assume the market value of securities purchased at a cost of $10,000 on margin (at 50%) increase in value to $12,000.  This $2,000 increase in market value would create SMA of $1,000, which provides the account holder the ability to either: 1) buy additional securities valued at $2,000 (assuming a 50% margin rate) without depositing up additional funds; or 2) withdraw $2,000 in cash, which may be financed by increasing the debit balance if the account holds no cash.   It should be noted that while an increase in market value over original cost creates SMA, a subsequent decline in market value has no effect on SMA.  SMA will only decline if used to purchase securities or withdraw cash and the only restriction with respect to its use is that the additional purchases or withdrawals do not bring the account below the maintenance margin requirement. SMA will also increase on a dollar for dollar basis in the event of cash deposits or dividends. More details at http://ibkb.interactivebrokers.com/article/66