The most important thing the manual will tell you is that
Allows for selective calling. You can "page" a specific radio or group without bothering others on the channel. icom icf121 manual
The official Instruction Manual Icom IC-F121 VHF mobile transceiver provides comprehensive guidance on operating the 50W radio, which features 128 memory channels and a rugged MIL-STD 810 construction. ICOM Canada Key Manual Resources Instruction Manual The most important thing the manual will tell
(p.11–13):