When the customisation variable completion-use-hotkeys is non-nil, you will be able to select a completion by typing a single character (the numerical characters 0–9 are used by default).
You will probably also want to display a list of the possible
completions, so that you know which completion will be selected by each
of these “hotkeys” (see Displaying Completions in the Echo Area,
and Completion Tooltip). By setting completion-use-hotkeys
pop-up, the hotkeys will only be enabled when the tooltip or
pop-up frame (see Pop-Up Frame) are displayed.
The following variables affect the behaviour of the completion hotkeys:
pop-up, the hotkeys are only enabled when the tooltip (see Completion Tooltip) or pop-up frame (see Pop-Up Frame) are displayed.
Of course, however many characters are in completion-hotkey-list, there cannot be more completions available than the number actually found! The maximum number to find is limited by completion-max-candidates, Miscellaneous Options.