Overview
The "unavailable funds" fee, not to be confused with the "non-sufficient funds" (NSF), "overdraft", "exceed hold" or "overlimit" fees, is a fee that results from a transaction that posts to a negative available balance and a positive physical balance, as applied to a Demand Deposit Account; usually a checking account. The fee is typically applied at the end of the business day, as most banks process transactions at the end of each business day.
An account has two distinct balances a posted balance or physical balance and an account balance. The difference is comes from transactions that have been applied to the account but have holds against them. To understand how an "unavailable funds fee" comes about it is important to understand the difference between the two types of balances.
Read more about this topic: Unavailable Funds Fee