Product reference
A product reference maps an item sold in an external online store to the corresponding PIM product in Customer's Canvas Hub.
For example, the Premium business cards item in your online store needs to open the correct personalization setup. Its product reference tells the storefront which PIM offering to use.
The reference makes the connection between the store and PIM; it does not define the available paper choices. Those choices can be tailored within PIM using a product link based on the Business cards product.