Yes the HSI CHG zone indicates how much of the available regen braking capacity you are using up.
When you press the brake pedal, you are requesting that the system provide braking power.
Having worked out how much braking power you are requesting, the system then finds out how much braking power can be supplied by the various braking systems (regen, friction and engine). It then decides which braking system does what with regen having priorty.
Most of the time, the regen braking system can provide all of the braking power requested. But there are times when it cant. Such as HV battery too full, HV battery too hot, not going fast enough, etc. At these times, you will shoot through the CHG zone very quickly.