Two categories of 406 MHz EPIRB: Cat I auto-deploy and float-free; Cat II manual deployment.
Regulatory detail & full definition
All maritime EPIRBs operate on 406 MHz (Cospas-Sarsat satellites) with a 121.5 MHz homer. Cat I is mandatory under SOLAS — float-free hydrostatic release at ~4 m depth. Cat II is supplementary or for non-SOLAS use. Battery life >= 48 h transmission. GPS-equipped EPIRBs (since ~2010) shorten location resolution from ~5 km to ~100 m.