Hi Bas,
Your question is about the conceptual change, where the “Stock classification” has moved from the (global) Product level, to the more specific Sales Channel Product-level.
The “Stock Classification” is the cutomer-facing description of the inventory of a product and thereby, the delivery promise. It typically has values like “In stock”, “In stock, delivery within 2 days”, “Currently out of stock”, “No longer available”, etc.
In multi-channel, multi-region commerce, we see that this promise is not globally valid for all channels or countries per product.
For example, a product needs to be installed by a specialized party who does deliver in the Netherlands, but not in Germany. In that case, the same product can be “Available for installation” on the Dutch website, and “Currently not available” on the German website.
Therefore, the Stock Classifications (the business rules) have been expanded to link with the Product Assortment, which in turn is linked to the Sales Channel.
It is stored on the Sales Channel Product to give the fastest way to retrieve products - see the explanation for normalisation and Sales Channel Product on this same forum: Best practise PLP / PDP - #2 by Joost