If you want to set aside inventory for a Partner, the best way to do this is by creating a duplicate product in Shopify. Brands only have to list this duplicate product on the Catalog for Brands sales channel, not their online store.
For ease of management within your Shopify, we recommend Brands title this duplicated product "For Catalog." Your Partner can edit this title later on.
Check the box for the new product to be sold on Catalog for Brands within "Sales channels and apps" in Shopify. After this, the Brand's product will be available to their Partner in the Catalog app.