From The Urban Dead Wiki
The UDID, ID or Urban Dead ID is a number identifying a character in Urban Dead. It, along with the character's name is a unique identifier, allowing the character to be identified from it. It can be used for certain actions via "question marks", such as, and most generally, profile links.
If you know a character's name and need to know their UDID, http://profiles.urbandead.net provides a database of all character's in the game, allowing you to search by name and providing the profile(s) link(s) to match.