Given there are no BDs (so they are theoretical) maybe there isn't a threshold defined? A WD will slowly cool to a BD over Trillions of years, so it's all line

Given there are no BDs (so they are theoretical) maybe there isn't a threshold defined? A WD will slowly cool to a BD over Trillions of years, so it's all linear. Funnily enough, they are probably every shade of red/brown in-between - but "brown dwarves" are usually reserved for objects that never where "proper" hydrogen-fusing stars, but are large enough to fuse deuterium.