Your available balance won’t include funds that haven’t been cleared or debited yet and any transactions that haven't been processed, such as cheques and recent contactless card payments. So, you’ll need to ensure that you keep track of these.
If the available balance is a negative amount, you won’t be able to make any further payments until you’ve paid money in.
Your account balance will show your balance after all your transactions have come off. Whereas an available balance tells you what funds are available to spend, withdraw or transfer immediately.
Your available balance could be more or less than your account balance. Your available balance could be more than your account balance if you have a planned overdraft. Your available balance could be less than your account balance because of un-cleared funds, such as a cheque that hasn’t cleared yet.