A swatch image typically shows the key difference between variation products in relation to a product variation attribute.
You can define swatches for variation products. For example:
Swatches are represented as a special view type together with image variations, and are maintained per base product. Typically, there is only one swatch image per view type and variation product combination. Stacked swatch images are not allowed. For example, products with the same color code, can have a different color tone. Swatches don't always represent color codes. For example, a swatch can represent highly zoomed detail images of the fabric. In this case, the swatches for red T-shirts, red knit dresses, and red silk scarves are different. By representing a swatch as a view type, every color variation product can provide the required swatch image.