It has nothing to do with the shape of the bodies, but the scope of the class Rob defined. It’s suppossed to be a GT class featuring GT cars. The DP is a prototype, and while it races on track with GT cars, it is a different class. USGT is supposed to be GTE, GT3, GT4 etc. The Pzero is based off of the Zonda, which did have a few privateer GT entries, so it fits the scope of the class. It’s not officially licensed, so it just looks similar. Just like how the Protoform PFM10 resembles a McLaren MP4-12C GT car. So while the Pzero looks like the DP, it represents a completely different class of car, hence why one is legal, the other is not.