Both the SDI and s-Video are considered internal inputs since they go thru the media server.
The RGBHV and the VGA are considered external inputs, as they feed directly to the projector.
The general idea behind having both an SDI IN and OUT are so that you can take a signal in from, lets say the DL.3s camera, and send that feed to other fixtures. In other words, you can essentially daisy chain DL.3 together with SDI.
