completion-base-position is a variable defined in `simple.el'.
Its value is

Position of the base of the text corresponding to the shown completions.
This variable is used in the *Completions* buffers.
Its value is a list of the form (START END) where START is the place
where the completion should be inserted and END (if non-nil) is the end
of the text to replace. If END is nil, point is used instead.