User attributes are used to create a relationship between a user and an artifact.
User attributes perform the following functions:
When you define a user attribute, you also indicate what permissions a project member must have to be able to view or edit the attribute. Optionally, a user attribute can be configured so that a user with this attribute is sent an email notification whenever someone modifies an artifact instance that uses the attribute. User attributes can be associated with the "To" and "CC" actions.
User attributes appear to the user as a list box with choices derived from the list of all users registered with the domain based on the base permission defined for the attribute. Refer to the What determines what a project member can do in Project Tracker? to determine the value of permissions held by the user in the project.