PLBs are only used in the case of an emergency. They send out an emergency beacon message with your location and then the rescue party comes for you. They are only one-way communication. You can’t communicate at all with them, and they are just for emergencies. But they also have no subscription plan after you buy the device. Bivy Stick transmits at about 1.6 GHz, and PLBs transmit at 406 MHz. PLBs do not use Iridium. They use completely different satellites and dispatch and rescue systems.
The Bivy Stick uses 1.6 watts of power to transmit.