Central Person Registry provides a means for associating information to user accounts in a pervasive manner. As ELMS:LN is a fragmented by design architecture this helps create a central record for the user. This is important when ELMS:LN is delivering the user login workflow to have a single account to relate information back to (like passwords even).
It has a user record and some simple profile data which it can store for referencing. Most people never experience this system, it's one of those things that helps other things work and account experience be seamless.
Eventually it will utilize JWT authorization tokens with oAuth2 and become an interaction point for authentication systems. It is positioned for this purpose though currently is not utilized in this way.