Yes, but pick only those seats that may be empty? Forgive me is this not correct, but my understanding is that everyone is assigned a seat by the system 24 hours before the flight (unless minors are involved, in which case that is 36 hours) and when check-in opens, passengers can see which seat they have been allocated and try to move or change this to any other unassigned seats. There may be no or very few other options (the bassinet, perhaps if there were no babies). Is this correct?

Or is it that everything that is not pre-sold is left open and passengers can choose what they like on first come, first served basis?