cmpl-prefix-entry-tail is an alias for `cdr' in `completion.el'.

(cmpl-prefix-entry-tail LIST)

Return the cdr of LIST. If arg is nil, return nil.
Error if arg is not nil and not a cons cell. See also `cdr-safe'.

See Info node `(elisp)Cons Cells' for a discussion of related basic
Lisp concepts such as cdr, car, cons cell and list.