A marketable limit order is a limit order priced at or slightly through the current market — a buy limit at (or just above) the ask, a sell limit at (or just below) the bid. It behaves like a market order in that it fills immediately against available liquidity, but it carries a hard price ceiling: it will never fill worse than your limit.
Why not just use a market order?
On options — especially wide-spread, thin-book short-dated contracts — a naked market order is dangerous. If the book is thin, it can walk through several price levels and fill far worse than the quote you saw. That's a blank check. A marketable limit gets the same speed but refuses to fill beyond a price you accept — capping the worst case.
The trade-off
The cap is also the risk: if the market moves through your limit before you fill, you get a partial fill or none, and you may have to re-price and chase. So the limit has to be set intelligently — tight enough to protect you, loose enough to actually fill in a fast tape. That balance is the whole art of short-dated execution.
A market order says "fill me at any price." A marketable limit says "fill me now — but not off a cliff."
How systems use it
A mechanical execution engine can set that limit against the live spread on every order, adjusting to conditions rather than using one static rule — pricing aggressively enough to fill without signing the blank check a market order represents. Managing that ceiling in real time, identically every time, is exactly the kind of unglamorous execution discipline that NoVo handles so a fill doesn't quietly eat the trade.