Returns a unique number identifying the user executing the application file in which the tag appears. If no user reference number was received (via the "_userReference" search argument or an HTTP cookie) when the application file was called, a new number is generated; otherwise, the number passed in is returned.
For more information on user variable tracking, see Chapter 8 of the User's Guide.
The user reference number can be used for reliable tracking of user variables.
userKey, altuserKey
<@USERREFERENCEARGUMENT>
<@USERREFERENCECOOKIE>