Passport Card is only for land and sea travel. Global Entry is for air travel and for all air travel you have to carry your passport book. Earlier I just possed the question if instead of inserting my passport if the passport card would work. I have yet to try it or ask CBP if its possible to use it just for identification purpose at the machine.

I have my passport card listed because I had it on there for my nexus membership and when I applied for Global Entry it just used my nexus application for my Global Entry application. I will research the topic and let people know what I find out.