Orderbook does not employ leverage through margin trading. Chances of total loss are close to zero.
Orderbook only trades spot BTC/USDT currently. Any loss in account portfolio balance means that the BTC held had lost value and unless they were sold off, losses are unrealised.
Portfolio Balance = BTC value + USDT amount
BTC value = Current Market Price x Amount of BTC in wallet
The difference in portfolio balance at 06:00 GMT+8 each day over different periods of time will be the calculated performance.