Top is a BDSM term for either a dominant partner in BDSM play, or a partner who applies stimulation to another (the bottom), and who may or may not be dominant.
A service top is a top who applies sensation or control to a bottom, but does so at the bottom’s explicit instructions.
The main difference between a dominant and a top is that the dominant does not follow instructions, although they are limited by what the submissive is willing to do, and does not necessarily give rather than receive acts. The top may sometimes be the partner who is following instructions, i.e., they top when, and in the manner, requested by the bottom. Contrast this with the pure dominant, who might give orders to a submissive, or otherwise employ physical or psychological techniques of control, and might instruct the submissive to perform the act on them.