percysmith If you've got reservations over that point I'd take your word for it!

Of course the airline(s) won't stipulate inventory in their T&Cs, so it was a personal observation:
- I've never been able ot upgrade when there wasn't inventory
- After upgrading, their inventory number reduces correspondingly
- I've had a waitlist upgrade (using miles) clear several weeks before a flight, without any inventory being made public - I'd guessed that they created one unit of inventory, but used that to clear my upgrade?
- CX redemption / upgrade inventory varies depending on membership tier