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