A service top is a BDSM term used to refer to a submissive who enjoys topping in accordance with their partner's desires. This person is the active person who's applying activities and techniques to a partner, but rather than dominating that partner is acting out that partner's wishes.
Some BDSM devotees disparage service tops for failing to maintain the traditional BDSM relationship dynamic.
More About Service Top
Most professional dominants are considered service tops when they engage in "pay for play" scenes, but non-professional dominants may also engage in service topping for both the bottom's enjoyment and their own. Service topping may also be done as part of a submissive's training by a dominant.
In most cases, the term is used in a disparaging way to refer a person who identifies as a dominant and still allows the submissive to dictate what will or will not happen in a scene. This happens when those who identify as dominants lack the courage, confidence, or conviction in themselves or their relationship to ask for what they want, or even demand or expect reciprocity in a scene.