In a BDSM context a service top is a dominant who is dominant because the bottom desires or even requests it. A service top does not need to be dominant in general. The bottom, in many cases sets the rules.
Most professional Dominatrices and Masters are considered service tops.
This article is based on "Service top" from the free encyclopedia Wikipedia (http://en.wikipedia.org). It is licensed under the terms of the GNU Free Documentation Licencse. In the Wikipedia you can find a list of the authors by visiting the following address: http://en.wikipedia.org/w/index.php?title=Service+top&action=history