The function uses the Great Circle method of calculating distances between two points on the Earth. The shortest distance between two points on the surface of a sphere is an arc, not a straight line. (Try this using a string on a globe surface.) Because of the curved surface, the angle from Point A looking to Point B will not be converse of the angle looking from Point B to Point A. The function Distance Between Two Sets of Coordinates sets of coordinates, shows this more clearly.
Bearings or azimuths start with 0 degrees toward true north, 90 degrees east, 180 degrees south, and 270 degrees west (clockwise rotation).
DXers (long distance listeners and viewers) may find this program useful to help determine the stations they might be able to receive, if an antenna is oriented in a particular direction.