I think when you stop having pronounced remittances and remissions and a lot of your symptoms stay and perhaps worsen in severity, then you are crowned SPMS..
RRMS is remitt. followed by Remiss., where you heal and pretty much go back to normal mobility.
SPMS is only a progression of RRMS..
PPMS is when you start out with an attack that never really stops and you progress more rapidly.
RPMS is when you have remittances and remissions, but never completely heal, as you do in RR, and progress more rapidly..
The above are my words and not to be confused with anything scientific..