I wrote a post some time ago trying to summarize the original article that put forth these four types. It's at
http://www.medhelp.org/posts/Multiple-Sclerosis/Types-of-MS/show/1045115. I think the types were largely done to create categories that they hoped would be useful for clinical trials. It's also important to note that they weren't based on any biological markers or other objective evidence, but rather were developed by consensus.
So far as I understand this, in order to have PPMS, you have to have from the very beginning ongoing progression or plateaus without remissions, although you are allowed to slightly improve at times. By definition, if you ever had a remission, you can't have PPMS.
Before they had these official categories, they sometimes used a category called single attack progressive (SAP). This was a single relapse and (often long) remission followed by ongoing progression that was considered to be similar to PPMS. I often think this is most similar to my situation, but according to the current definitions, the progressive part of this course would be considered SPMS.
If you had ON and it got significantly better and you sustained it (I think from the definition you would have to sustain the improvement for a month for it not to be counted as part of the same event) then you did have a remission. It doesn't have to go back to normal to be a remission; it just has to improve. Exactly how much I'm not sure.
If you start out with RRMS (even just one relapse) and then start to have ongoing progression without periods of stability at a baseline (or mostly without?), then I think they say you have SPMS. This is why it's called secondary.
Some people with SPMS have superimposed relapses and some people don't. If you don't have superimposed relapses, I am not sure there's much practical difference between SPMS and PPMS. There are some neuros who think SPMS is the same thing as PPMS except that the RRMS part has been truncated off. Then there are some who think they're two totally different processes. Who knows?
I don't think the difference particularly has anything to do with how bad your symptoms or problems are, but whether or not you are having inflammatory relapses definitely affects your treatment options.
I think maybe you need to talk to your neuro some more about what this means for you in particular.
I hope this is somewhat useful.
sho