In the Hotkey’s section of the manual, it says:
Ctrl-u Starts a prefix entry sequence
What does this mean? I can’t find any other reference to it.
Ignore it. It needs to be removed from the manual. This is a left over remnant of years ago, when Ardour took keybinding inspiration from the audio editor “snd”, and thus from the text editor “emacs”. Its been gone for quite a long time.