The credit Overdraft Limit is effectively a loan of credits automatically set by the system.  The amount is based on the frequency and sizes of your jobs & purchases over time.  You can see it in the Envoy Profile page:

In this example, the Credit Overdraft Limit is set to 5 [1].  When your credit balance reaches zero any  waiting jobs will be stopped, but running jobs will be suspended.  The "loan" provides you with extra time to top up your account to prevent those jobs being stopped and loosing any frames that were running.

When you top up your account, any credits loaned will first be paid back, and any remaining credits purchased will then be added to your account. For example, suppose your credit overdraft is 5, and you have consumed all 5 of those credits. Then, if you were to purchase 20 credits, 5 will be used to repay the credit overdraft loan. The balance, 15 credits, will then be added to your account.