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.