Let's examine the purpose of a contract, a legal instrument, in theory and in practice. A contract is always a formal statement of trust, and as drawn describes the extent of the parties' mistrust of future events --voluntary and involuntary-- which could, if one or more occur, impair or foreclose fulfillment of mutual obligations. A contract anticipates and defines executable relief from harm attributable to such events.

Let's examine a generic definition of the credit instrument provided by securities brokers to prospective buyers. Various terms of the agreement proferred --including but not limited to closing share price, buyer's reserve requirement (unobligated capital requirement), minimum margin requirement, and timely payment of service fees-- constitute a contract, an agreement, between them.

Buying on margin involves taking out a partial loan from one's broker in order to cover a larger investment than one's capital could directly cover. A margin call most often occurs when the amount of actual capital the investor has drops below a set percent of the total investment. A margin call may also be triggered if the broker changes their minimum margin requirement --- the absolute minimum percentage of the total investment that one must have in direct equity*.

Note that effective demand for payment (to seller, to broker, to reserve account) rests exclusively at the discretion of the creditor/broker. Uncertainty (mistrust) about valuations, the buyer's liquidity or portfolio exposure informs the creditor's decision to alter terms of margin agreements outstanding. Timing/schedule of such emendments however may or may not be stipulated in a contract. I don't know, but I would imagine such limitation is a marketing feature that differentiates brokerages' range of services as well as individual client risk profiles.

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*I suppose, sales-speak for dividend bearing securities. <urp. ahh> See investopedia.com

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