Understanding Your Swing Metrics: Shoulder Tilt Explained

In a golf swing, the shoulder tilt angle refers to the angle formed by the shoulders in relation to the ground. It measures how much the shoulders are tilted or inclined at various stages of the swing.
The shoulder tilt angle is of great importance in a golf swing for several reasons:
- Balance and Stability: The shoulder tilt angle is critical in maintaining balance and stability throughout the swing. By achieving the correct tilt angle, golfers can distribute their weight effectively, ensuring a centered rotation and preventing excessive lateral movement. This stability allows for consistent and controlled body movement during the swing. 
- Swing Plane and Path: The shoulder tilt angle helps establish the appropriate swing plane and path. The swing plane refers to the imaginary inclined plane around which the golf club should move during the swing. The tilt angle influences the club's path relative to this plane. A well-measured tilt angle ensures the club travels on the desired plane, promoting consistent ball contact and minimizing slices or hooks caused by incorrect swing paths. 
Face-On

P1: Address
Tracks how much your shoulders are tilted toward the ground at the address (P1), measured in degrees relative to the horizontal axis.
P7: Impact
Tracks how much your shoulders tilt toward the ground at the impact (P7), measured in degrees relative to the horizontal axis.
Example
If the P7 angle is 41.6°, the angle formed between your shoulders and the ground is 41.6° at the impact.
Further reading
https://www.youtube.com/watch?v=8qQ2zGQ7VRU
Down-the-line

P4: Top of Backswing
Tracks how much your shoulders tilt toward the ground at the top of the backswing (P4), measured in degrees relative to the horizontal axis.
P8: Release
Tracks how much your shoulders tilt toward the ground at the release of the club (P8), measured in degrees relative to the horizontal axis.
Example
If the P8 angle is 53.7°, the angle formed between your shoulders and the ground is 53.7° at the release.
Further reading
https://theultimategolfschool.net/shoulder-tilt/