This is mostly correct, but in the second case, you don’t have to scan in; the window just has to open. So if you book at 9:00 and the window opens at 9:45, you can book at 9:45 whether you’ve used the first one or not. Keep in mind that the 9:45 opening will be good until 11:00 with the “15 minute” grace period.
This can be an important distinction depending on your strategy.