What does 50%/50% mean? 70%/30%? When is this useful?

Are you thinking of brake mixing, or perhaps throttle-trigger travel?

If it’s throttle-trigger travel, some radios over the years have allowed ~70% throttle – 30% brake (or reverse) travel, or 50-50. Think it started with the old resistor wiper arm speed controls, which were 70-30 or thereabouts.

If I’m not mistaken many transmitters are just 70-30 these days. Should provide a little better or finer throttle control, at the tradeoff of brake/reverse of course.