Let's look at the following example. Alice wants to sell 1 synthetic BTC for 30000 USDC and pay a maximum amount of 5 USDC as a fee for the trade.
Bob has a vault with -1 BTC and +31000 USDC. Notice that Bob's position can be matched against Alice's order. If Bob's position is below the maintenance margin according to the system parameters, the application can match between Alice's limit order and Bob's position without his signature or any consent from him.