It's not being answered so will give my own situation. In Dom/Sub.
Domme, I'm still learning, I can do it with my partner but most men don't want the bf/master in the room understandably, and also have a duo partner to mentor me if the client is willing, otherwise the situations I have been in, I had to ask a lot of questions before the appointment to see if I could fill the role they were looking for. But if I have a domme power over someone, my inner beast refuses to ever let them get the upper hand. They could try, but will just be met with being shown their position is as the sub.
Sub, not so much. It takes a gfe visit first to see if the gent has the kind of personality and vibe I am willing to be submissive with. I very much find most men who call themselves Doms don't deserve the submission and unless it comes naturally from me, it just isn't going to happen, their in for a powerstruggle fight. Trying to be submissive if I don't feel it for the person, has me hating them by half way threw the visit and I just want it done then. So I refuse to agree to an initial first time visit as a sub, untill I know the gent gives the right vibe I need, to enjoy the submissive role with him. (It is a role I often keep for my relationship, as it takes a lot of trust for me.)
It isn't something I can play games with. It is why it depends on the chemistry I have with the gent. Upon meeting the first time for gfe, I know what position either of us has in the who's more dom or sub. Switching just isn't in me. I can't just treat it like a game. It is what it is within us already. I am really no good with playing pretend.
But I tend to think when someone writes dom/sub, they mean they can play either role. Its not necessarily both roles in the same visit, but most who offer both might likely, unlike me, be able to just switch easily.