Exactly as it means. If there's no availability in T class for upgrade, you would not need to buy an M class ticket. The AM agent is being nice to try to save you money. A TA (or CX reservation) can help you hold the underlying reservation for a period of time before you have to issue the ticket. You can use this period of time to find out if there's T class availability for upgrade. I don't think AM is allowed to book the upgrade seat anymore without the underlying economy ticket issued, but it will help them to check the inventory. If you book via a TA (not via CX reservation) and coordinate the time well, you can do this: 1) Book the M class ticket; 2) Call AM to check T class upgrade availability; 3) If there's T class upgrade availability, have the TA issue the M class ticket; 4) Ask AM to apply the T class upgrade; 5) If for whatever reason this couldn't be done, ask the TA to void the M class ticket. Do note that "void" usually needs to be done within the same day or within 24 hours depending on where the TA is located. The TA may or may not charge you for void.

Also, before you use the miles, check the fare diff. between M class and E class to make sure you think it's worthwhile to use AM for upgrade,