That's a tricky question to answer, but generally its determined by advancement of symptoms, do they regress or simply progress and how quickly do they progress, and how long they regress (if they do). Another factor they might include is the response to DMD (disease modifying drugs) like copaxone, or tybirsa (sp?). And at times who knows if they're not just spinning a wheel and tossing a dart to determine it .

Just remember doctors, regardless of what they may believe, aren't gods nor prophets, and ms is still a bit of a guessing game. If your rolling straight on to rock bottom, PPMS is likely what ya got but it could surprise you with a sudden stop and backing up (RRMS) or it may simply pause for a bit (SPMS). And then there's the wonderful "clinical isolated syndrome" which means you look like a duck, quack like a duck but we're still not sure how well you'll taste in orange sauce.