Where does the recommended pitch come from?

In case you are using a fixed structure, the software recommends a pitch value that will ensure at least 4 hours of unshaded operation of the PV arrays during the most unfavorable day of the year (winter solstice).  
 
If you’re using a tracker, the recommended pitch distance is the one that makes the GCR 40%. 

For E-W structures, the pitch distance is calculated as follows: 

1. pvDesign calculates the minimum pitch distance: the one that makes clearance distance 0.  
2. It then adds an industry-standard clearance distance value:
   1. For 1H or 1V: 0.3 meters
   2. For 2V or 2H: 1 m
   3. For other configurations: 2 m