So what does <Off Air In> mean, then? I could hook an antenna to a converter box and run it through the D12 using the same input on my tv?
It doesn't mean that there is an OTA tuner in the D12. This is a holdover from the days when:
---(1) TVs that only had a single coax input.
---(2) Directv didn't offer locals in many markets.
The coax passthru allowed people to get locals and Directv channels without having to switch out the coax or installing an A-B switch. I had this setup on a D10 in the pre-SWM days, but I don't think it would work on a SWM system...