Wonder if this will help you. Or maybe you know this much already and want further answers. If so, I'm sorry.

Relapsing-remitting MS is defined by inflammatory attacks on myelin (the layers of insulating membranes surrounding nerve fibers in the central nervous system — CNS). During these inflammatory attacks, activated immune cells cause small, localized areas of damage. Progressive forms of MS involve much less of this type of inflammation.
Primary-progressive MS (PPMS) is progressive from onset without any inflammatory attacks. People with RRMS tend to have more brain lesions (also called plaques or scars), containing more inflammatory cells, while people with PPMS tend to have more spinal cord lesions, containing fewer inflammatory cells.
Secondary-progressive MS (SPMS) follows after an initial relapsing-remitting course. Of the 85% of people who are initially diagnosed with RRMS, most will eventually transition to SPMS, in which the disease begins to progress more steadily (although not necessarily more quickly), with or without any relapses (also called attacks or exacerbations). This means that there is less inflammation and more gradual destruction and loss of nerve fibers in SPMS than in RRMS.
Progressive-relapsing MS (RPMS) is a progressive disease course from onset, characterized by gradual destruction and loss of nerve fibers, but with occasional inflammatory attacks as well.
In RRMS, women are affected 2-3 times as often as men; in PPMS, the sex ratio is 1:1.
RRMS is diagnosed earlier than the progressive disease courses.
Most people with RRMS are diagnosed in their 20s and 30s, although it can occur in childhood or later adulthood.
The onset of PPMS tends to be about 10 years later.
Because the relapses that occur in PRMS follow an initial period of disease progression, a person may be diagnosed with PPMS before the PRMS is identified.
The transition to SPMS generally occurs in people who have been living with RRMS for at least 10 years.