In Site Administration, you can specify the default catalog for each site. For every request, Commerce sets the customer’s catalog profile property to the current site’s default catalog. This mechanism ensures that the customer will always see a catalog that is correct for the current site (see the Commerce section for more details on how this mechanism works).

You set the default catalog for a site using this option on the Operations tab:

When specifying a default catalog, you can choose to create a new catalog or use an existing one. If you choose to create a new catalog, you must subsequently populate it with categories, products and SKUs in Merchandising. Multiple sites can have the same default catalog (for example, in Commerce Reference Store, CRS Store Germany uses the same default catalog as CRS Store US).