Thanks Dux, I hadn't seen that.
To make a long story short, I porked my test terrain and called Skuzzy and confirmed that all fields need to be set as Group Masters and the Object Data field determines if they're Airfields, Vehicle Bases or Ports. He walked me through the steps, I'm still learning something everyday.
IIRC I've had trouble setting the Object Data field when Group Master was first selected so it's something I want to look into while building a new test terrain from scratch. It's about time I build a new one anyway, my old one is a mess.
And yeah, I wonder about that dropdown list too, having it active when you select Group Master would confirm you're supposed to use Group Master and set the field type. Maybe in the next revision...