In looking at the Garmin 430 install manual I came across terminology I'm not familiar with. For Connector P4001 analog outputs to the HSI they use the following to identify pin outputs: 21 Main+left, 22 Main+Right (2.5 V common), 23 Main Lateral+Flag, 24 Main Lateral-Flag (2.5V Common), 25 Main+TO, 26 Main+From (2.5V Common). I know these are the outputs for respectfully left and right course indicator output, Nav flag in view and out of view, and To/From indications. I am stumped by Main and the 2.5 volt common. Is the 2.5v a reference voltage that varies up and down depending on the condition? What does Main refer to? Thanks in advance.