Any idea what certificate they would refer to? What happens if I don't end up showing them anything? Can they cancel my reservation?

I've seen these as well....no firm idea; possibly and internal incentive rate. Perhaps Chris can help out.

The worst that can happen is that you'll pay the standard room rate. These incentive rates, when I've seen them, aren't much better than the AAA, Ent. or Encore rates, if better at all. Personally, I'd book a rate which I understand and know applies to me. That NPR code is a catch-all.

Good luck!

Pat