I wasn't really interested in how the number was determined but more what the number meant. When I reached out I asked how the weight capacity was determined for a particular model, so I am making an assumption that if it's calculated a certain way for that model then the company uses the same standard across all model of kayaks under its label/brand.
One thing nearly all manufacturers mentioned was that as you approach and potentially exceed the weight capacity you won't actually sink the kayak, but you will begin to negatively impact its paddling characteristics. Even after reaching out and getting responses I am a firm believer you should try not to exceed 70% of the listed max weight capacity of the kayak you plan on purchasing.
Anyways here are the responses for weight capacity:
Brands: Bonafide, Hobie, Jackson, Pelican, Perception, Vibe, and Wilderness Systems
Weight Capacity = Published Weight Capacity
Brand: Bass Pro Ascend
Weight Capacity = Published Weight Capacity - (Seat + Accessories + etc)
Example: Ascend 12T's true weight capacity is roughly 340 lbs (350 lbs - 10 lbs for seat & rod holder)
Brands: Hurricane, Ocean Kayak, Old Town, and Native Watercraft
Weight Capacity = Published Weight Capacity - (Weight of Kayak + Seat +Accessories+ etc)
Example: Native Watercraft's Titan 10.5's true weight capacity is 379 lbs (500 lbs - 121 lbs for kayak, seat, drive, etc)
As of right now Feel Free (Joy Sports) has not responded. I'll update when I get a response.