The term top is not to be confused with Dom which is the person who "directs or controls the scene".
A male Dom might enjoy CBT and tell a sub what they are to do to perform on them to give them pleasure. In this case the Top is the submissive (following the direction of the Dom who controls the scene) and the bottom is the Dom (receiving the sensations given of the top). This need not necessarily be in terms of strictly BDSM activities. A dominant may also enjoy bottoming for such things as a massage or valet service and ordering their submissive to top for such activities.
A service top might be a d-type, m-type, or s-type that performs topping skills on a willing bottom primarily for the benefit of the bottom (as a service to them). A professional service top is generally referred to as a pro dom.