Customers available Credit amount

How is the total of the customers available credit calculated when credit checking is invoked? Does it only look at AR items or does it also include uninvoiced orders?

If a user wants to capture a new order for a customer, how can they tell what is the customers available credit ? If they know the amount of available credit and the amount of the customers credit limit, then they can tell if the order will go on credit limit hold or not before they place the order. Is this amount stored somewhere in the database?