Quite sure it just uses _NET_WM_PID and fails if it isn't there. At least as far as I remember I had issues terminating my own application with xkill until I s

Quite sure it just uses _NET_WM_PID and fails if it isn't there. At least as far as I remember I had issues terminating my own application with xkill until I set the _NET_WM_PID myself, the framework I used only did the bare minimum to get a window on the screen.