The manual says it's energy recovered (ie, not something relative to system capacity), and this seems consistent with the other indicator that shows instantaneous braking power with a distinction between what the regenerative system is doing vs what (if anything) the traditional brakes are doing.

which car? i think the number you're referring to is relative to electrical system capacity.

It's NOT relevant to overall (tyre, brake system & engine braking & regen braking) braking system performance since that's a dynamic value variable over many factors constantly.