Eclipse puts out 2 versions of certain head units that are essentially identical but with different model numbers. Like the AVN5510 and the AVN52D. The "D" models are sent to certain select installers/distributors.
I would call eclipse tech and ask them for the exact differences between the 6620 and 62d. I found out the avn76D was the same as the 726E, except with 3 volt rather than 5 volt pre-outs, and the 76D doesn't have the esn security feature.
Not sure about the newer units, but on the older ones.............like the AVN5435 vs. the AVN30D.............They are mostly the same, but the 30D only has high-level outputs and no separate sub control. The "D" models are geared toward the OEM replacement market through select car manufacturers and dealers.