completion-kill-region is an interactive compiled Lisp function in `completion.el'.

(completion-kill-region &optional BEG END)

Kill between point and mark.
The text is deleted but saved in the kill ring.
The command C-y can retrieve it from there.
/(If you want to kill and then yank immediately, use M-x copy-region-as-kill.)

This is the primitive for programs to kill text (as opposed to deleting it).
Supply two arguments, character positions indicating the stretch of text
to be killed.
Any command that calls this function is a "kill command".
If the previous command was also a kill command,
the text killed this time appends to the text killed last time
to make one entry in the kill ring.
Patched to remove the most recent completion.