A lot is a quantity, not an amount of money. What the exposure rule measures is the face value that quantity represents, so one standard lot of a pair trading at 1.0850 is $108,500 of exposure while one lot of gold is a hundred ounces. Multiply price by contract size by lots and you have the position value; divide it by your account size and you have the percentage this one trade adds. The cap tightens as accounts grow because a larger account moving the same percentage moves far more money. Exceeding it issues a warning rather than closing the account, but two warnings end it, so the number is worth knowing before you click buy. Every limit is published in the trading rules.