Not as complicated as you might think...
I identify as a submissive Top. I am submissive within the relationship and dynamic, but I have the ability to Top should it be required by my dominant. Darcy considers himself dominant and allows me to top on occasion. Ultimately it is his choice.
I do not switch roles, but I submit to the 'moment' as desired (if that makes any sense).
A dominant can be any 'role' - bottom, masochist, sadist, top etc - as can a s-type.
Switches can make very wonderful D/s couples or poly relationships.
Peace