I think its a combination of when you signed up and what you put for an estimated finish. Here's why. I signed my dad up in 2014, but after the first day of registration don't remember exactly when, but at least a day or two later, maybe more. I put 15-16 min/mile for estimated finish. He got corral D. This year I signed up myself and my daughter both the first day of early DVC registration. I put my estimated finish at 10 min/mile. I got corral B. I also signed my daughter up during early registration and entered 15-16 min/mile (she's six) and she got corral C, as did my mom who signed up that day and entered the same time. The times for my parents and daughter were the same 2014 to 2015, it was when everyone signed up that was different. I also signed my daughter and I up at the same time, her first because I knew my mom was already in, and it was more important my daughter get in than I, and I ended up in B, she C (but I had a faster estimated finish). So I do not think it is solely on registration time. So I think its by time first, but toward the end of each corral filling if they have a lot of people with the same estimated finish, it then goes by order of registration.