Of course, in neither case did they disclose what "sell-out" means, precisely.
The maximum number of units hypothetically available by that date?
That seems to be the criteria based on US sales. Everyone who ordered by 8/1 @ 8:00 PM PDT is in the first shipping group. There have been successively later dates given to people who ordered later. Either they have
n number of units available to ship in the first group or they can only ship
n units per day. Seems like Amazon is limited to number of units available to ship in each group, since the later ship dates have jumped by 2-4 days or so rather than one day. I don't think there's anything hypothetical about it, just physical limitations.