@betsy789 Your hip crease at the same level as the top of your knee is what makes a rep count for squats in my opinion. With barbell squats some people can go loeer and some don't.
Also, powerlifters usually don't go that much lower from what I have seen. Probably because it requires extra energy. Plus, it is easier to injure yourself. Why? Everyone has a point where your lower back will start to round. With proper technique most people can reach parallel with barbell back squats. However, lots of people will experience their back rounding lower than that. For some it may be lower, but powerlifters lift very heavy weights. When you have a lot of weight on your back it is harder to control it and you may not notice if during some reps you went lower than you should have gone. So, parallel is safer in my opinion.
Now, the point where you stop also depends on the type of squat. With goblet squats I can go as low as I can without putting pressure on my back.
So, if you are training for hypertrophy and can go lower than parallel for barbell back squats without rounding your back, then I think it is fine. If you use heavy weights be careful though, because as I said, when the weight gets heavy you may not notice if you are exactly at the right spot.