Imagine instead of pulling a detection zone from upper left to lower right, you could instead select any one of 144 ‘blocks’ (16x9) to look for motion. That’s what “Added support for a custom motion detection zone” is about.
If you don’t see this, my guess is either your firmware or software is not current? Below is my V3 capture is from production app 2.19 and V3 firmware 126.96.36.1990 on iOS. Yes, there is a newer firmware version available, but I am not on it yet. So is the newer 188.8.131.52 bad?
BTW, you say PIR which is “Passive Infrared”. The Outdoor cam has a PIR sensor, but the V3s still use pixel movement.