Any items that you don't specify as returnable through your Return Groups will not be returnable on the storefront.
For example: you select products A and B as returnable, through a return group called "Returns", but product C is not selected. If a customer were to purchase all three products and wanted to return them, they would be able to return A and B on the storefront, but not C.
Note: Visit What are return reasons and statuses? for more information.