If we are talking about the same thing, it is as Philippa said above. A mix of XC and show jumping. It's actually really cool! I rode one years ago with my little Arab. I feel like if the horse excels in one of those two events and is a little iffy about the other, they feel more confident as they go through the course. If that makes sense :P It's a really good "fun" event, not to be taken too seriously. Fun school day. :)
I saw an event once that was labeled derby cross or something very similar, it was indoor though, but it looked like stadium jumping with some XC jumps in the mix. You had your standard rails / poles that could be knocked down, but also an equal mix of jumps that were stationary like huge logs, stumps, etc.