EPIRBS are for use in maritime applications. The 406 MHz EPIRBs are divided into two categories. Category I EPIRBs are activated either manually or automatically. The automatic activation is triggered when the EPIRB is released from its bracket. Category I EPIRBs are housed in a special bracket equipped with a hydrostatic release. This mechanism releases the EPIRB at a water depth of 3-10 feet. The buoyant EPIRB then floats to the surface and begins transmitting. If you own a Category I EPIRB, it's very important that you mount it outside your vessel's cabin where it will be able to "float free" of the sinking vessel.
Category II EPIRBs are manual activation only units. If you own one of these, it should be stored in the most accessible location on board where it can be quickly accessed in an emergency.