PLBs are used only 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 1-way communication. You can’t communicate at all with them and are just for emergencies. But they also have no subscription plan after you buy the device. Bivy Stick transmits at about 1.6 GHz. PLBs are 406 MHz. PLBs do not use Iridium. They use completely different satellites, and dispatch and rescue systems.
The 1.6 GHz or 406 MHz is the frequency at which the signal transmits. Iridium is much higher than PLBs but that doesn’t mean stronger transmission. It’s just like a different lane so that the signals don’t step on each other. Higher frequencies tend to require a line of sight more.