I’m going to post here pretty much the same answer I’ve just posted on a similar issue by Kim.
The types of MS have for years seemed to be set in stone. If you have a clear pattern of relapses and remission, then you have relapsing remitting MS. If you then move into a phase where remission stops, then you are secondary progressive. If however, you never have clear cut relapses and remission, then you are progressive from the start, or primary progressive.
But, there is a lot of debate about this. For example, many people suffer smallish relapses with remission for years before they are diagnosed, by which time they are progressive, they are then labelled PPMS.
Other people who are classed as SPMS continue to have relapses. And neurologists are often reluctant to describe someone as SP because while they are still classed as RR, they continue to be eligible for DMDs.
There is now a growing consensus that MS should not be divided in the way it is. See this article from the excellent Barts Blog (I think PJday has posted this or something similar before as I know she agrees with it) Multiple Sclerosis Research: Advanced MS
The authors are arguing that MS is one disease, not three, and instead of the word Progressive, we should be using the term Advanced. This then allows people with RRMS to be classed as Advanced although not progressive. And removes a lot of confusion. There is so often a feeling when you have had RRMS for years to start feeling like you might now be progressive. And then you have a really serious relapse with some clear remission, which proves you wrong. It can lead people to stop taking DMDs too early.
In my case Iain, I was diagnosed with RRMS in 2002 after having MS for 5 years. I continued to be treated as RR for most of my years with MS, even after it seemed clear to me that it had become progressive. The first time my neurologist used the term progressive in relation to my MS was after I ran out of options for DMDs. (I had bad side effects from too many.)
This was my neurologists way of ensuring people were treated with DMDs for as long as possible. But that was his opinion as to how to treat MS, it is not the same with every neurologist.