EmmyTheHucow: At what weight do you go from being a BBW to a SSBBW?
I think that you have to take height into account. A woman who is 6' need to weigh more than someone who is only 5'2. BMI is a reasonable way to take that into account as long as the individual isn't hugely muscular.
My personal opinion is that the SSBBW limit is somewhere around BMI 50, which is also the limit for superobesity.
According to your profile, you are 5'6", so you will become superobese at 310 lbs.
If you feel that you want to aim higher than 310 lbs, you can try to become super-superobese, which is BMI 60 and 372 lbs at your height.
Yes, I think its a bit subjective. The term was invented to distinguish between how different life can be for people who are VERY fat, compared to regular fat.
Plus size shopping sucks, ordering everything online is harder. Fat in a car is hard, but having trouble getting in and our is complicated. Do you have to scope out restaurants for their seating types before arriving? Do you get many looks and comments on the street? Extender on a plane, and researching second seats? Worried about job interviews because you're not just 'fat' but 'clearly have something going on-type-fat'? Issues fitting in and navigating all sorts of spaces on the whole?
Generally, this different type of experience is what inspired people to use the term 'SSBBW' to note someone who is at a size that forces them to deal with life in a different sort of way. For most people, this is around 400-450 pounds, but for some it might happen a few pounds earlier or later.