What is KEY_KP0 to 9 of Linux input event code definitions?

I noticed that there are KEY_KP{0-9} ten keycodes in the evtest output *"Supported events"* section, but I don't know what they are. I'm looking for a few keys to use for a particular keyboard(Chromebook function keys) and bind the function in the desktop environment. I've used the event's definitions (such as KEY_REFRESH) as much as I can, but some functions like *Fullscreen* and *Overview* aren't defined here. So now I'm going to use f{13-24} to implement them. although I'm still curious what's the KEY_KP is. By the way, I'm using [keyd](https://github.com/rvaiya/keyd) .