A top is not necessarily dominant, and vice versa. The term only defines that the person is on the giving end — in other respects, it is neutral. It should be noted that the top is often the partner who is following instructions given by the bottom. This practice is also known as "topping from the bottom".
There is also a verb to top which means to play the top's, active, decision-making and controlling, role. This is the form used in topping from the bottom, for example.
The verb's logical counterpart, to bottom, also exists but is used less because the bottom's role is more of a passive one.