Deck 6 is the old traditional dining setup. They have one seating at 5:30 and one at 7:30. You are booked for either early or late seating and you are at the same table every night with the same people (if you want to sit at a larger table.) You can not just walk up and get seated. You have to sign up for this type of dining before the cruise. Deck 7 is for walk ups.
It is not a problem if you want to go to a specialty restaurant any night but it is common curtesy to tell the wait staff know the night before so they aren't waiting for you. It just means that you have a standing reservation every night but the time is locked in.
There are crowds outside deck 7 between 7 and 7:30 because the line is right outside the elevators and stairs. And people were confused about where to go the first few days. Folks were trying to get to deck 6 and were in the line for deck 7 dinning. After a few days they learned not to wait in the line but go to the stairs and walk down. The first few days the line stretched down the corridor past the Wheelhouse bar. But like I said, it went fast and the line was gone by 7:30 or 7:45.