I don't know what it's used for, but I had a retailer want to enter it as the security code (as that is apparently how some Visa/MC's do it). The number on my card had faded too much to read. Since every other retailer had used the 4 digit number on the face, I suggested that this retailer use the 4 digit number, and it worked fine. In speaking with Amex Customer Service later, I mentioned the faded numbers and asked if I needed a new card; the person responded no, that those number are not used by the cardholder or retailers. I still don't know what they are, but apparently they really don't matter to us.