Every trading disaster story we hear contains the same sentence: "I had a stop-loss, but it didn't execute." Nine times out of ten, the trader used the wrong order type. This is the least glamorous topic in trading and one of the most expensive to learn by experience — so let us learn it on paper instead.
Both SL and SL-M orders sleep at the exchange until price touches your trigger price. Suppose you bought a SENSEX call at ₹500 and place a stop with trigger ₹478. Nothing happens until the premium actually trades at or below ₹478. The difference is entirely in what wakes up.
An SL order carries a second number — a limit price, say ₹476. When ₹478 triggers, the exchange places a limit order: "sell, but at ₹476 or better." Here is the trap: option premiums in a fast fall do not walk down one tick at a time — they leap. If the next trade after your trigger is at ₹468, your limit order at ₹476 sits unfilled while the premium keeps sinking. You now hold a losing position with a stop that already fired and missed.
An SL-M order carries no limit. When ₹478 triggers, a market order fires: "sell now, at whatever the best available price is." You might fill at ₹477, or in a violent move at ₹470 — a few points of slippage. But you are out. For an option buyer whose worst case is the premium going to zero, certainty of exit is almost always worth a few points of slippage.
Two honest reasons. First, in thin, illiquid contracts a market order can fill absurdly far away — a limit protects against a freak fill. Second, option sellers and spread traders sometimes need price control more than exit certainty. Note that exchanges have at times restricted SL-M on certain contract types precisely because of freak-fill risk — check what your broker currently permits.