Upgrades using miles are for the most part only possible for a carrier’s own flights. While it’s easy to book award tickets using miles on one airline for travel on partners, upgrades are a much trickier beast.

There are several exceptions but those are pretty cumbersome. For instance Star Alliance upgrade awards are usually for one segment only and require the passenger to be traveling on a full fare ticket. There are plenty of carrier-specific restrictions but as a broad generalization this holds. You can upgrade many fares or even all fares using some airlines’ miles, but you want to upgrade on another airline you need to be traveling full fare.

With American miles you can upgrade on British Airways or Iberia, but it’s one class of service only (so coach to premium economy on BA only, or premium economy to business) and you need to be on a full fare fare.

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Delta has allowed upgrades on Air France, KLM, and Virgin Atlantic. And now they’ve added upgrades on Aeromexico using SkyMiles to their program terms and conditions.

• For transborder flights you can only upgrade on Aeromexico using Delta miles if booked on a Delta codeshare.

• You can upgrade Aeromexico domestic flights within Mexico using SkyMiles only if your journey begins in the U.S. and you’re traveling on a Delta codeshare.

• Upgrades are permitted on Delta Y, B, M, H, Q, K, L, U and T fares.

My general recollection is that Aeromexico upgrades have booked into the same (“O”) inventory as award tickets although readers will correct me if I’m mistaken.