Your Commerce instance initially has a single site, or store; you can, however, run multiple sites from a single Commerce instance.
Each site corresponds to a store and can have its own catalog. For example, if your company has multiple brands, you can have one site per brand. You could also have a site whose catalog includes items for sale in one country and another site whose catalog includes items for sale in a different country.
This section discusses what you need to know before running multiple sites in your Commerce instance.