Sorry, my link did not appear above, it's slippage, so if your order cant be placed at your exact price it will, the deviation is the maximum price difference it will be placed if your order is behind a big queue of orders to be filled by the broker and by the time your order gets filled the price will have moved.
https://ctrader.com/forum/ctrader-support/2723
Below is an explanation of slippage.
https://www.investopedia.com/terms/s/slippage.asp
If your order is filled at the intended price then there was no slippage of price and no deviation is used.