The external completion style is used with a ``programmable completion'' table that gathers completions from an external tool such as a shell utility, an inferior process, an HTTP server. The table and external tool are fully in control of the matching of the pattern string to the potential candidates of completion. When external is in use, the usual styles configured by the user or other in completion-styles are ignored. This compromise is for speed: all other styles need the full data set to be available in Emacs addressing space, which is often slow if not completely unfeasible.