RM or Rights Managed means the buyer is buying a license for a specific image for a specific purpose for a specific time. They expect it to be exclusive for that purpose/time. I.E. for a book cover.
RF or Royalty Free has no exclusivity expected (it can be used by multiple licensees at the same time) although the agency may or may not expect it to be exclusive to that agency.
If you sign up as exclusive with an agency the terms depend on the agency. Some expect all work done by the artist to only be sold via them. Other consider exclusivity on and image by image basis.
