A product definition can include product properties and product variant properties. For example, the "Book" product definition in the Retail site includes product properties and also product variant properties, while the "Generic Product" definition only includes product properties with no product variant properties.
If a product is created based on the "Book" product definition, and no variant product is created, the following message occurs when you attempt to view the item in the catalog:
The product you requested is not currently available through this catalog.
However, if the product is created with the "Generic Product" product definition, the item is viewable in the catalog.