The Assigned MP is the MP in the primary site that the client has selected.
The Resident MP is the one the client agent is actively communicating with.
The Proxy MP (even though Proxy MPs don't actually exist anymore) is the MP in the secondary that the client agent is using if it's within the scope of a secondary site.
To my knowledge, the resident MP and Proxy MP will always be the same if the client is within the scope of a secondary site.
Jason | http://blog.configmgrftw.com | @jasonsandys