The FOV types are:
Hdeg 4:3 - Horizontal Degrees based on 4:3 aspect ratio.
Hdeg Res - Horizontal Degrees based on your configured resolution and aspect ratio.
Vdeg - Vertical Degrees.
Hrad 4:3 - Horizontal Radians based on 4:3 aspect ratio.
Hrad Res - Horizontal Radians based on your configured resolution and aspect ratio.
Vrad - Vertical Radians
Multi - Multiplier
Since games use different types, you can't simply set the FOV in e.g. R6 to 90 and expect it to be the same as in CSGO. That's where the option to change the FOV type comes in. If you for R6 select the same FOV type as CSGO (Hdeg 4:3) and enter 90, the output will tell you the FOV you need to configure in R6 to get the exact same FOV as in CSGO.
The FOV type will automatically change when you select a game to match the type the game uses.