Comment by brod_ie

In the ideal scenario yes but they don't have to be: you can use Onin as a modern, E2EE personal calendar on your own. We're seeing the most common pattern in the private beta for people to plan on existing chat services and then move the event into Onin once arranged, effectively replacing the legacy email invite system of current calendars. From there the Onin event becomes the destination for the continued planning/chatting/sharing photos etc.