I recently converted from the Citibank AAdvantage Visa to the AAdvantage World MasterCard. The materials that I had beforehand explained what silverpie described above - you still have an assigned revolving credit line (which in my case remained unchanged since I converted an existing account). They let you charge some amount over this each month, but that amount is due in full on the next statement, as well as the minimum payment on the revolving credit line. I called the credit department to find out just how much over the revolving limit they let you go, and I got the impression that it was about 25% over the revolving credit line. So, it's essentially a revolving credit line that lets you go over the credit limit by some amount with no "overlimit" fee.