Probably can't give clear and concise explanation but anyways I'll try not to confuse even more.
Perhaps the easiest way to obtain the credits is within the ODEleven application - after you have purchased, activated and are logged into your Pro account. The app takes you to Google Play Store for the payment and go from there.
EDIT: Actually I'm not sure that you even need the Pro account (the 24.99€ activation thingy) for this.
I'd guess you can do essentially the same through purchasing at website - the credits will be linked into your Pro account.
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The credits are needed to use so called "Apps" within OBDeleven. The Apps which are just "one click functions" to code or adapt things. For the nerds using Apps there is no additional value. Everything can be done using coding and/or adaptations of the control modules themselves.
Then re T6 - at the moment there are not many Apps for T6 (just 4+2). Actually I'm not sure if any of them works on T6 - except the oil service and inspection service resets - of course. But as they can be reset using MFSW/stalk buttons not much added value there.
The above applies also to T6.1 - at the moment.
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However, for more modern platforms than T6 there are more Apps already available, e.g. even old Golf (MY2014) more than 70 Apps. But again - doable through codings and adaptations.
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Anyways, the Apps is just a glimpse of OBDeleven's capabilities. The real power is in other areas...
etc, etc,...
And, just a glimpse (1 of 3 screens) of deep waters of T6 EU6 diesel engine functions - probably not everyone's cup of tea
.