Is there maybe a link by PostNL (the national postal carrier) that explains these rules that we can use as a source?

I checked their website, and I cannot find anything on there that explains it. There are plenty of other resources on the web that explain the rules.

If you search "postcodekaart" (postal code map), you will see lots of maps. You can easily notice they never start with zero.

The letter combinations "SA", "SD" and "SS" are avoided because of their associations with the Nazi occupation.