BigTime lets you setup several cost center types within the system. Cost centers are like specialized custom fields, and they can be setup to fit your firm's specific needs. Every project and every staff member can be linked to a combination of cost centers.
The advantage to using cost centers (over standard custom fields) is simple: every report and virtually every "view" screen in the system can be sorted/grouped by cost center. So, if your firm has 3 different offices, you should setup a cost center type called "office" with values of "Chicago, New York and LA" Then, every project list report can be sorted and grouped by "office" (so the LA projects group together, the Chicago projects group together, and the New Your projects group together).
You can setup your cost center TYPES (e.g. - what they're called) in the Tools...System Settings...Vocabulary screen. The actual cost center values (e.g. - the pick lists) are setup in the Tools...System Settings...Cost Centers screen.