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:
Tracks how much your shoulders are tilted toward the ground at the address (P1), measured in degrees relative to the horizontal axis.
Tracks how much your shoulders tilt toward the ground at the impact (P7), measured in degrees relative to the horizontal axis.
If the P7 angle is 41.6°, the angle formed between your shoulders and the ground is 41.6° at the impact.
https://www.youtube.com/watch?v=8qQ2zGQ7VRU
Tracks how much your shoulders tilt toward the ground at the top of the backswing (P4), measured in degrees relative to the horizontal axis.
Tracks how much your shoulders tilt toward the ground at the release of the club (P8), measured in degrees relative to the horizontal axis.
If the P8 angle is 53.7°, the angle formed between your shoulders and the ground is 53.7° at the release.
https://theultimategolfschool.net/shoulder-tilt/