The balance with which the users can place a bid is the bid balance. It is contingent upon total balance and if the user is engaged in any active/ongoing auctions.
Ex: Users have 100 USD in their account. It has already participated in two bids of 30 USD each. Now, he cannot participate in a bid of 50 USD.
If a user has 10 ETH in their account, and they have already participated in two bids for 3 ETH each, they cannot participate in any bid that exceeds 4 ETH even if the bid amount hasn’t been used for purchase. The 4 ETH is the bid balance of the user.