Just checkin [
]. I think what you are refering to is the plug in for the optional RMX-1 module, which operates a remote control. You can turn on the system by remote from the field for diagnostics and whatnot. I am not too familiar with Rainbird, but that is what I assume the port is for. This is a link for information about it.
http://www.rainbird.com/landscape/products/controllers/rmx1.htm