Perpetual contracts and traditional futures contracts are similar products, but with a key difference. There is no expiration in perpetual contracts and is available for trading permanently.
In the traditional futures market, the contracts are marked for delivery at a given future date and thus have an expiry. In essence, the asset must be delivered as per the contract when the futures contract expires.
Perpetual contract, on the other hand, mimics the spot market with an intention to reduce the futures price and the mark price gap.
*Mark price - it is the value of the futures contract during trading hours. It could sometimes vary from the actual futures market price in order to protect traders from market manipulation.
It is calculated using the below formula:
Mark Price = 30 seconds EMA of (Futures Market Price - Index Price) + Index price
The same price is used to calculate unrealized P/L of trades.