I'd like to know also.
A couple of years ago a group of us had 5 sites in the 100 loop and we had one family that needed to stay in a cabin. I tried to get one of those cabins and was advised they were not available for ressies.
When I have inquired at the Fort about them I usually was told they were VIP cabins, but I have seen people using them that clearly weren't what I considered the VIP type. Even though I stay in 116 a lot of the time, which is right next to them, I have not questioned any of the people staying in them because I thought it might be rude.