According to NMSS:

"The name for this disease course comes from the fact that it follows after the relapsing-remitting course. Of the 85% of people who are initially diagnosed with relapsing-remitting MS (RRMS), most will eventually transition to SPMS, which means that the disease will begin to progress more steadily (although not necessarily more quickly), with or without any relapses(also called attacks or exacerbations)."

From what I understand, many neuros are reluctant to say officially that someone is now SPMS mainly because (last I knew) having that diagnosis tanks the ability to stay on DMDs.

I've researched the transition from RRMS to SPMS a bit--I was diagnosed with RRMS over 7 years ago, with symptoms going back 12-13 years. I suspect I'm SPMS now, and I suspect that my neuro suspects that also, though he hasn't said it in so many words.

In my research, I notice that the descriptions of symptoms overlap quite a bit and are fairly subjective. An exacerbation, for example, can be new symptoms, or it can be a worsening of old symptoms. And then, of course, there are pseudo-exacerbations. I'm getting very gradually worse--no true exacerbation for years. But is that because I'm SPMS, or just because it's a long time between relapses?

I could be wrong (imagine that), but I think it's pretty arbitrary.