It isn't super secret, but VW likes to keep it that way as much as possible. I guess it is intellectual property which protected from public scrutiny would keep those of us of the unwashed masses out of nasty places like under the hood. VW wants only dealer techs to be doing skilled jobs like changing light bulbs, afterall.
The reality is that the more dependent you are on dealers, the more money they can suck out of your wallet. Never mind the extra funds they charge due to mis-diagnoses and sloppy labor that breaks parts. But the other part of it is, if you pay for a subscription, you can get the latest and greatest. Result: VW screws their own dealers, too!