Allow me to over-explain.
Early is fixed, upper dining. Main is late, upper dining. Upper refers to the upper dining room. That's deck 3 on most ships, or 5 on the Volendam and Zaandam. You report to your assigned table at your assigned time.
Open seating is one deck below. It's the same menu, but you report to the host stand any time during dinner hours and are seated when a table in your preferred size is available.
Occasionally, fixed seating people might wind up in the lower dining room, or vice versa, depending on capacity needs.