What does the B mean?

"Braking" - as in "Engine Braking" - downshifting to slow down without using the brakes.

I understand "B" increases regenerative braking returning more energy to the battery and saving wear on the brakes. I use it a lot when I see a traffic light change to stop when I am a distance away or am going downhill faster than I like.

I understand that leaving the shift in "B" does hurt mileage (when crusing) as it is equivalent to leaving a normal car in a low gear. However, where is it documented that using it when slowing or going downhill does not increase mileage by increasing the battery charge rate over leaving it in "D" and riding the brakes?

I looked at the reference and found:

"B" mode also increases regeneration current to 30 - 40 amps with no feet on
the pedals, so the part about "more regeneration" is somewhat true.

Given this, the knowledge (I can hear) that the electric-motor generator spins faster when decelerating in B (compared to D), and that normally there is battery capacity available to take in the higher charge, I remain of the opinion that using B for deceleration (with or without a light touch on the brakes) normally saves fuel by returning more charge to the battery than using D with with or without a light touch on the brakes.