Just sharing an experience in case anyone is interested here.
I have an EZY flight on Thursday. I checked in online last night to see what seat it allocated me, and I got 'D'. An aisle. I wanted a window and I wondered in what order they would allocate seats, logically the least desirable (middle?) first in order to encourage people to pay for an allocated seat. I was a little surprised to get an aisle as although I hate the aisle, it seems most people want that.
There is obviously some algorythm determing how seats are allocated in the DCS. On conventional DCSs, generally it starts at the front, allocates the window seats down both sides, then the aisles, then the middle seats. Groups/families are of course seated in the same row.
Having got my boarding pass, I cannot now change that seat on line, but only when I check in at the airport, and by paying for the selected seat. I am wondering how that will work in practice.