Xavier: I think you can go either way, as long as the domain is an activity or process that you want to "exclusively" control; I am not sure if that would be a valid domain as written, as I wonder "what is the activity or property you are looking to control".... one time we had a domain which was something like "pricing for services, except private trainings" or something. I don't know that it would be the "best" solution, though perhaps that is all the clarity you have. And if no one objects, it could serve as a valid domain.
In either case, the role w/ a domain must create policies or allow others to impact that. So I would try to stay focused on the tension, and if you know what / how you want to constrain something you can try a circle policy to start, or a role domain and let the role figure it out.
Try not to address all the related tensions, and it can inform if you want a role domain or a circle policy.
Sometimes there is no clear answer, and you just need to try something.