The Zero Price is the price at which the liquidation of the position would result in the account balance becoming zero, including all fees. This is calculated based on the assets in the account at the time of the liquidation and adjusted for the liquidation fee. 

If the liquidation process executes all of the liquidation orders at the Zero Price then the account balance will be zero at the end of the liquidation. If the orders are able to be executed at prices better than the Zero Price then this benefit is kept by the customer.