Personalizing a product, it's good idea to visualize it at different personalization step. It works with mockups. A mockup is a special image, which represents a real product. Mockups allow a customer to see a design not only as a flat image, but on a product. For example, how it looks on cups, T-shirt, and so on.
You can use mockups in two different approaches: Product Specification and the Product Information Management Module.
In Product Specification, you can use a mockup at each personalization step. If your product doesn't have options, you can use one mockup file while customizing in Design Editor or at the preview step.
Product Information Management Module allows a customer to create product variants. It can be a t-shirt with color or artwork variations. In such products, you can connect many mockups for the product, and each of them can be used with different designs. In the PIM module, the thumbnail mockup type appears. It uses for displaying product when selecting variants.