This is a shortcut for switching to a third level of the active layout (if it has one) which is handled by X. org. It allows modifier-only shortcuts.
Dit is in fluchtoets om te wikseljen nei it tredde nivo fan de aktive yndieling (as it ien hat) hokker hannele wurdt troch X. org. Jo kinne allinne in modifikaasjetoets as fluchtoets ynstelle.