Margin
Collateral a broker requires against positions with obligations — chiefly short options. Requirements scale with the risk of the position and can rise sharply when markets move against it.
Options margin is not borrowing in the stock-margin sense; it is a performance bond. Defined-risk structures like verticals require roughly their maximum loss, while undefined-risk short options require formula-driven amounts that grow as the trade goes wrong.
The operational danger is that requirements expand exactly during stress, forcing liquidations at the worst prices. Professionals size to survive the requirement spike, not the average day.
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