Categories are another level of organisation, allowing you to group users together in an additional way. This means that even if users are split over different units or organisations on the Unicorn LMS system, you can also group users together by categories. This allows more options when carrying out mass registrations of users to activities.
For example, if you want to assign a course to all the administrators on the system, for instance, but they are scattered across different units, as long as they are assigned to the Administrator category, you can register them in one go instead of having to go through and registering them individually.