why do only some systemd command completions work?

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http://forums.fedoraforum.org – Suppose I do this (where <tab> means I press the tab key for force a completion) Quote: systemctl ena<tab> ---> completes to --> systemctl enable then Quote: systemctl enable chro<tab> --> systemctl enable chrony-wait.service In the second one, the systemctl completion for "chro" only offers me "chrony-wait.service" but doesn't offer me chronyd.service which would seem to be to also be a valid completion for the fragment 'chro' why is that? (HowTos)