Originally Posted by
californiadreamin'
NEXUS is a creature of Canada, the US, and several Caribbean countries and is not limited to citizens of those countries. It is not accepted as the sole document of entry to the US except at US preclearance stations.
Nexus is accepted as the sole document of entry to the U.S. I've made countless land crossings and a couple of water crossings using only the Nexus and I've entered the U.S. at airports when entering from other countries although that probably fails under the GE portion of the program.
Regarding Air Canada the document you booked with is the one they want to see whether it is Nexus or a passport.