Thanks to Drifter and his questionable lines- I am a pro at this question! haha
A TB/QH can only be registered as an appendix unless it wins enough points to be qualified as full QH. Drifter is actually a third generation appendix, but both his sire and grandsire won enough ROM points on the track to be classified as "QH" and not as an appendix.
Like, Drifter's grandsire was an appendix who was bred to a TB, but it shows up as QH to TB on Drifter's papers since AQHA granted him "full QH" status. So the resulting colt (Drifters sire) was an appendix who then won "full QH" status just like his father. HE was then bred to a TB to get Drifter.
So drifter is listen as 50/50 but in reality he is about 10% QH and 90% QH.
AQHA is so weird. I first noticed this when I was looking through Drifter's lines and was like pulling my hair going "WHERE IS THE QUARTER HORSE?! YOU ARENT A QUARTER HORSE!"