Since a bidder's user info is anonymous, this tag will contain the real ID value only for that bidder, and the seller of an item that the user is bidding on. For all other users, the real ID value will be replaced with the anonymous value, according to these rules:
When bidding on items listed on the US site: UserID is replaced with the value "a****b" where a and b are random characters from the UserID. For example, if the UserID = IBidALot, it might be displayed as, "I****A".
Note that in this format, the anonymous bidder ID stays the same for every auction.
See the Field Index to learn which calls use HighBidderUserID.