I think I found the answer while on vacation in Japan.
I just spent a week driving a Toyota Aqua. In the US this car is known as the Prius C.
Here is the gear stick.
On the Prius the 'B' mode setting is for Engine Braking.
It is explained here - Questions about B Mode
Use the B position of the Mode Selector Lever in exactly the same circumstances as you would change to a lower gear to slow down a conventional car. If you are going down a long and steep hill, controlling the car's speed using only the brakes can cause them excessive wear, get them hot and may also tire your foot. The solution in a conventional car is to select a lower gear. The engine then spins faster and the energy it takes to do this holds back the car. Less brake pressure is needed to maintain a slow rate of descent. The Prius, however, has no gearbox in the usual sense of the term. Toyota have made special provision for engine braking and to activate this you put the Mode Selector Lever in the B position. By a special configuration of the motor/generators, the engine spins and helps hold back the car just as in a car with a gearbox.
....the purpose of B mode is to throw energy away by spinning the engine.
On a Prius the 'B' mode setting saves the friction brakes from overheating when going down a hill, by using the electrical motor (in reverse) to slow the car down. The recuperated energy cannot all be saved in the battery, so it is 'wasted' by spinning the engine. The Prius has a small HV battery ~1kWh and cannot recuperate more than about 10kW.
The 'B' mode setting was copied by Nissan for the Nissan Leaf, and then Kia copied it from Nissan. But on a fully electric car this setting is not the same thing. It cannot be Engine Braking because a BEV has no mechanical engine into which energy can be 'wasted'. There is no longer a need to 'waste' recuperated energy because it can be saved into the battery. The Soul EV can recuperate more than 43kW.
What the 'B' mode setting actually does on a BEV is alter the level of recuperation on the accelerator pedal. It is about one foot driving. This mode is far better altered on the I oniq EV using paddle shifters. Hopefully the next generation of Kia EV's will lose the 'B' mode too.
( a different topic - the Prius gear stick being set diagonally is a much better design than the Soul EV which is straight. Just like the Soul EV if you accidentally push the stick into 'P' while driving; a mechanical b olt will be pushed into the transmission to stop it. This destroys the transmission. It's much less likely with the diagonal configuration than the straight.)
Lol - I just noticed how stupid this website is. It translates my use of the word metal 'b o l t' into the car 'B o l t'.