Trading on Delta Exchange happens within the 'allowed trading band' which is defined as the -/+2.5% range around the current Mark Price. For buy trades, the upper bound (Max Price) of the trading band is applicable. This means any buy order cannot execute at a price greater than Mark Price + 2.5%. Similarly, for sell orders, the lower bound (Min Price) of the trading band is applicable. This implies that the execution price of a sell order cannot be less than Mark Price - 2.5%. You can see the complete details of the allowed trading band and its implications here.