The 30m antenna has azimuth limits of 60 and 460 degrees. Azimuth 360 degrees is due North. Therefore there is an overlap range approximately toward East-Northeast, which the antenna can reach at a low azimuth 60 to 100 (from the South) or at a high azimuth 420 to 460 (from the North).
SET TOPOLOGY LOW selects to use the azimuth range 60 to 420 degrees.
SET TOPOLOGY HIGH selects to use the azimuth range 100 to 460 degrees.
Note that SET TOPOLOGY only has an effect for sources with an azimuth in the overlap range; when a source is in the azimuth range 100 to 420 degrees SET TOPOLOGY does not matter.
Compare Figure 4.