I have also seen this code in the comment section of the reservation, but only when the reservation was made by the Diamond Desk. Each of the reservations that had this code were for Hilton hotels in Europe. I don't know if that matters. Does anyone know what is the significance of this code?

The code used was GPM U: XXXXXXXXXX
(The X's represent numbers that I do not want to post here, but the numbers are the same on all reservations) Perhaps it is a reference number for the CSR from the Diamond desk. Just a guess.