I think the answer to this question is going to vary from person to person. I also think it depends on one's state of mind. For example, some people may consider me to be SSBW. I don't concider myself to be a SSBW as I can still by clothing in a regular plus size clothing store (max. size 28). I also think that even regular size women can have curves.
Ok, I just deleted the part that may sound like I'm preaching something (so not me and not what I want to do- lol).
I know I have not answered your question, but I hope it gives you some insight to what you maybe hoping to hear.
BBW and SSBBW are only different when you make them different,i consider myself SSBBW because i am tall,like I am 5ft 9 and a half i fit clothes in the store still 22/24 but it all depends on what you want to call yourself, Some women over 500 lbs consider themselves simple BBW's it all depends on you, im comfortable with either it does not matter