The Horizontal Sight Angle is the measurement at a point of the viewing angle towards the outside. The openings are the windows looking towards the exterior of the building. The horizontal sight angle of a room completely closed is 0°, the horizontal sight angle of a flat surface without any wall around is 360°.
The Horizontal Sight Angle indicator is used in particular by the European standard EN 17037 (in UK: BS EN 17037:2018 Daylight in buildings). We have designed a special module according to EN 17037 requirements for the horizontal sight angle, see this page.