Haier HCI604TG3 Instruction Manual page 34

The five zone model is also divided into two groups. In group a, if using boost on
one zone, first make sure that the other zone is set to power level 5 or below.
When group a is in flex mode, both zones can be set to Boost function. In group b,
you can only use Boost function for one zone at a time. If you try to use the Boost
for two zones at the same time, the first zone which is using Boost function will
drop out of boost function and switch to power level 5.