The human inhabitants of the world are split into the following categories:-
One Worlder
People who do not Step. For convenience this category will be subdivided by their world.
Two Worlder
People who have stepped between worlds.
Parallel
People who exist in both at the same time (I fear I may have made this needlessly complicated and will probably simplify.
The Fae are able to step at will between the known worlds, subject to the power of their belief that they can.