The one thing I've wondered about is how you would check-in 24 hours prior. You need to scan your ticket or Octopus Card to get into the airline area. Fine. But then you check in your bags, and need to get out without actually getting on the train. So you need to scan your ticket or card again.
Not sure how that works.
One time I mistakenly went into that area without intending to board the train. I don't even remember why. All I remember was that I then needed to go to customer service to have them fix it.