this is such a subjective question lol but it def depends on the place cuz bbw to some people is just girls with a big ass like ice spice. In feedist spaces though it definitely is a much higher weight.
I would say around 220-230 is when a girl goes from thick fat to just straight up fat. it shows at different weights depending on height. I feel like it's where a girl's figure is typically "ruined" (improved) and they're just big af. At 200 girls can still seem chubby (of course when i was growing up a 200 pound woman was comedically large which is fucking insane)
for SSBBW it's even more subjective. I remember people saying it was 300 10+ years ago but i think inflation (no pun intended) of feedee's weights has pushed it more to 350-400. I feel like it's more of a size where a girl struggles to fit into society. Can't fit into booths, chairs break, leery of small bathrooms, can only shop at plus-size stores, maybe they start to eye the mobility scooter at the grocery store... it's definitely the size when someone goes from the fat you see on a daily basis to eye-catchingly fat. Clear the way type of fat.