Any transactions that are not made directly from your Reserve will be paid from the available balance. If the transaction exceeds the available balance, but not the overall account balance, the available balance will go negative, your Reserves will remain untouched, and your overall balance will reflect appropriately. You will then need to either release funds from your Reserves or reassign the transactions to a Reserve to bring your available balance back to $0 or positive.
Please note that POS (debit card) transactions and external withdrawals will always come from the available balance. You can reassign transactions to be paid from the Reserves after they have posted.