While a top and bottom tend to engage BDSM-wise only for specific scenes, a dominant and a submissive commonly have a longer-term agreement regarding authority given by the submissive to their dominant. In other words, a top must seek permission from their partner for each engagement. A dominant has long-standing authority to engage in a range of activities with their partner without needing to seek permission each time.
Thus, if a dominant feels inclined to---say---tie up and flog his partner, all he needs to do is order her into the dungeon or bedroom and get started. A top would need to discuss this and get agreement beforehand from his partner.