I don't really know for certain. My guess is that for USNA there are 2 sub-agencies, NAPS and foundation. The only thing I could guess is that USNA submitted new information to DoDMERB that he is neither of those, so that would make him "Main". Again, this is just a guess. Give me a little bit and I'll get the info "straight from the horse’s mouth".
That would make sense. He received a "scholastically qualified" letter from USNA. And from what I think I'm understanding from other threads, he is not eligible for consideration for NAPS or foundation.