Last price is the price of the most recent trade on the venue. Mark price is an estimate of fair value derived from spot prices on external exchanges. They’re usually close. When they diverge, it matters which one your venue uses for what.
Why two prices exist
Last price can be manipulated. A single large order into a thin book can print a price far from where the asset trades elsewhere — for one moment. If liquidations keyed off that print, every leveraged position on the venue could be attacked by anyone with enough capital to move the book briefly.
Mark price defends against that. It’s computed from a basket of external spot prices — a weighted median across major exchanges — so a wick on one venue’s book doesn’t move it.
What each is used for
Last price: what you pay when you trade. Your fills happen against the actual book.
Mark price: what your position is worth and whether it’s liquidated. Unrealised profit and loss, margin ratio and liquidation triggers all use mark.
The practical consequence
A wick on your venue that isn’t reflected across the broader market won’t liquidate you. A move in the underlying spot market will, even if your venue’s book hasn’t caught up yet.
It also means your liquidation can trigger at a mark price you never saw trade. That’s not a bug. It’s the system working as designed.