Both 'D' and 'B' modes regenerate energy to the battery, provided the battery isn't "full." It's just that 'B' mode does more of it — it gives more braking effect, some of which is regenerative, and some of which is true "engine braking." Also, once the battery is "full," neither mode will regenerate at all. In 'D' mode, all braking will be by the (lossy) friction brakes; whereas in 'B' mode, the ICE will be spun by the wheels (via the two motor-generators) to give (lossy) engine braking. So, with the battery "full," all braking is lossy.

Stan