The Horizontal Sight Angle is the measurement at a point of the viewing angle towards the outside.
The Horizontal Sight Angle indicator is used in particular by the European standard EN 17037 (in UK: BS EN 17037:2018 Daylight in buildings).
The EN 17037 standard uses 3 thresholds for the sight angle: 14°, 28° and 54°. This extension has to designed to follow eactly the norm recommendations. You may use the more general Horizontal Sight Angle module if you do not want to follow the EN17037 norm.