To control the permissions of users against 'Tracker', 'Category' and so on.
User A and B only can raise 'Feature' tracker.
User C and D only can raise 'Bug' tracker.
Also for category, A and B belong to UI department, so A and B only can raise 'Feature' tracker for UI department.
If A and B also belong to Database department, so in the category drop-down list, they can choose 'UI' or 'Database'.