If the Available Balance is less than the amount required for the top-up, then all of the Available Balance will be used to replenish the margin of the position. It is also important to note that Auto Margin Top-up cannot convert currency on its own and can draw margin only it is available in the settlement currency. For example, if the position that is about to go into liquidation required USDC margin and you only have BTC in your available balance, margin replenishment will fail.