In a workshop a swatch is a cutting from the bolt, pinned to a card with its name and composition. Online, a swatch is a small square image beside a product, one for each colour or cloth on offer, that a shopper selects to change the variant. Both serve one purpose: to show the material apart from the garment.
A swatch image has to be true to the cloth: photographed flat, in even daylight, straight on and close enough to show the weave, with at least one full repeat of any pattern. A physical swatch remains the reference where the exact shade matters, because screens differ from one another and from the fabric.