I think the confusion with these roles comes from the breakdown from EA:
Base: The player has access to all roles for their preferred position.
Role+: The player will be more effective in this role than others.
Role++: The player will be among the very best in this specific role.
I can promote a 65-rated player from my youth academy though, and train him to get a ++ role within a few weeks.
How I am able to get a 65-rated player a ++ role if that is supposed to mean they are "among the very best in this specific role?"
It seems the + and ++ really just indicate a ratings boost when played in a specific role to indicate a players experience and comfort level in that role.