Billable work by category

We have several departments that will be interacting with clients through ServiceDesk. Most of the departments' work is not billable and should not go against their contracted hours. We have one department (professional services) whose work is almost always billable and will count against their contracted time.

The work done by the professional services department is identifiable through several different methods. All their work will be under a single category with multiple subcategories and items. They are all part of a group that identifies them as professional services. All the requests they get should use a single template.

I am trying to figure out what is the best way to achieve this.