You've just about covered all bases. They have recently developed so called accumulator events in which players can play as many flights as he/she likes and bring all surviving chips through. Different tournaments and casinos
/websites have varying regulations regarding re-entry into each flight and if there's a limit to how many flights you can play
Most events it seems today allow players to play as many flights as they want but only take their biggest surviving stack. Sometimes if you have even 1 surviving chip, you can't enter another flight. Sometimes you have to forfeit your chips to be able to enter another flight.
In certain events though, like the WSOP
Main Event, there are no rebuys/re-entries, so you can only start a single flight. There is no try again, you just pick which one to play.
And day1a...day1f are the same things as flights. They may be on the same day, spread over 6 days, or something in between (say 1 per day but 2 on the 5th day).
To recap, the best thing to remember is that every venue and even every tournament is different, and you should always familiarize yourself with the exact rules and regulations before playing.
With that said, it seems the majority of flighted events are as you described in your first paragraph, in that you can enter as many day 1's as you like, and possibly re-enter/rebuy (as applicable) into the same day 1, and then take your biggest stack forward. The only caveat being you may have to forfeit a surviving stack bwfore playing another flight depending on the rules, which is again why I say you should know the exact rules before playing.
I hope I've been accurate and thorough. Granted I did just right this on a train going through the British countryside.