RP-Initiated Logout is functionality used by all three of the logout mechanisms. It is currently in the Session Management specification. It is needed for other logout mechanisms even when Session Management logout isn’t supported. It would be clearer to developers if we moved RP-Initiated Logout into its own specification.
Note that there are also separate RP-Initiated Logout certification profiles, distinct from the Session Management, Front-Channel Logout, and Back-Channel Logout profiles.