Metapackages allow you to group an extension that consists of multiple packages into a cohesive unit. This works exactly as described in standard composer.json documentation. If you have an extension that uses more than one package you must use a metapackage as the root package. Otherwise you should not use metapackage. A metapackage that you submit to Magento Marketplace should be a .zip file containing only the metapackage composer.json file.