completion-in-region is a compiled Lisp function in `minibuffer.el'.

(completion-in-region START END COLLECTION &optional PREDICATE)

Complete the text between START and END using COLLECTION.
Point needs to be somewhere between START and END.
PREDICATE (a function called with no arguments) says when to exit.
This calls the function that `completion-in-region-function' specifies
(passing the same four arguments that it received) to do the work,
and returns whatever it does. The return value should be nil
if there was no valid completion, else t.