Good question. So the credit goes into a pool that is your balance. In our eyes, with $100 credit, you have a balance of "negative $100." Usage accumulates toward 0 and once past that, a positive balance is the amount owed. So if you start with $100 credit and use $110 worth of service, you have a balance of "positive $10" and you owe $10.
You can use any and all of our services to go toward your usage. Any amount of credit that you do not use within 60 days will disappear. If you continue to have services with us after that 60 days, or after the $100 is used (whichever comes first), continued usage will be billed at the beginning of the next month.
Hope that makes sense :)