Trading mechanics
Margin allows a position with a larger notional value than the cash set aside for it, magnifying both gains and losses.
Required margin is the amount set aside to support a position; it is not a fee and should not be treated as a cap on loss. The position responds to price movement across its full notional exposure, so a small percentage move can produce a much larger percentage change relative to margin.
Open profit and loss changes account equity and therefore the margin available for other positions. Several correlated trades can concentrate risk even when each one appears modest in isolation. Position size, stop planning, and a reserve for adverse movement should be considered together.