Site factors: In general, site factors assume you are measuring radiation on a horizontal surface ( ie radiation measured using properly levelled cosine-corrected sensors) so you should make no adjustment for this.
However, there may be circumstances when you want to look at radiation load on the canopy floor, when it is appropriate to take account of the slope. You can do this by changing the Intercepting Surface settings. The values you have to enter are the Azimuth and Zenith angles of the perpendicular to your surface.
The Azimuth angle is the angle from North (via East) of the steepest downwards slope.
The Zenith angle is the same as the maximum angle between your slope surface and horizontal.
Select a single sided surface (double sided is only appropriate to single leaves that receive radiation from both sides).
Your values of radiation above and below will now be the same as if you measured them with a cosine corrected sensor in the plane of your surface.