Cost Control?
 

On my most recent Avis reservation I noticed that "Cost Control: B" was noted on my reservation under Personal Information in the confirmation e-mail.

Does anyone know if this is a positive or negative?

most often cost control is used by the booking company or your employer to tell your own accounting office where to code the costs to. its a cost center, and avis usually has nothing to do with it. If its a prepaid or partially prepaid through any means, it may have a cost control number on it. If your company booked it for you for work purposes, then its probably telling your company "bill it to section b" or some other coding process that your company is familiar with. its basically a black slot that companies or agencies can use to put information that will help them process it faster.