If your account's assets fall below regulatory and firm "maintenance" margin requirement, there will be a maintenance call.

Firms set margin requirements of their own—often called "house" requirements—that can be higher than the margin requirements under Reg T or the rules of FINRA and the exchanges.

For instance, a firm may set the maintenance margin at 30 or even 40 percent of the current market value of the securities in your account. Furthermore, firms can increase the "house" requirements at any time and are not required to provide you advanced written notice.

If your margin account falls below regulatory or firm maintenance margin requirements, you will end up with a maintenance margin call. In this case, you must put more money into the account to meet the required level, or the firm will likely force the sale—or liquidation—of some or all securities in your account to bring the account's equity back up to the required level.

A firm is not required to notify you of the sale, though most do so as a courtesy, nor does the firm let you choose which securities or assets are sold to meet a margin call.