I'm no expert seaman, but according to what I've read, the green segments are for monitoring for distress signals on frequency 2182 kHz and the red segments for morse code distress calls. The outer red segments surrounding the dial are guides for a radio ...
The red 500 frequency is no longer used…the green is and analogous to channel 16..having said that with satellite and epirb that is pretty old tech..love the dial…perhaps a slightly better depth rating for a marine watch.