Well, since I'm pretty Ace, that doesn't come up usually. But it goes something like this in our relationships: A top is the person doing something and the bottom is the person having something done to them, but it doesn't exactly equate to a submissive or a dominant because those are slightly different concepts. So a submissive could still be a top at least temporarily, because the dominant wanted it, or the submissive felt like they could fit that role, even though they are not inherently a dominant.

Also there's middles who top another person because a dominant told them to.

Edit: It gets messy when you throw switches and versatiles into the mix too because often they can't have any fixed roles.