The initials indicate the launch mode A: air B: multi-platform C: ground F: single person R: shipborne U: submarine launch.
The second letter indicates function I: air interception G: opposite strike.
The third type is M: missile.
For example, AIM is an air-to-air missile (such as a rattlesnake), RIM is a ship-to-air missile (such as a standard series), FIM is an individual air defense missile (such as Stinger), AGM is an air-to-ground or air-to-ship missile (such as Mavericks and Hellfire), BGM is a multi-platform attack missile (such as Tomahawk, which can be launched on the surface and underwater, or on the roadbed, just for service), and UGM is generally a submarine-launched ballistic missile.