In the Controller probe -> "Setup" tab -> "Misc." tab, there are 2 fields, "User tag 1" and "User tag 2" :
The user tags info are recorded in UIM DB, in CM_NIMBUS_ROBOT table, inside the "user_tag_1" and "user_tag_2" columns.
These fields are defined as VARCHAR(256). So they can accept alphanumeric characters, with a limit of 256 characters.