Directory for extra content. That isn't in the WordPress repository. This can be themes and plugins, but too WordPress eXtended RSS (WXR). That is the file format to import WordPress content using the WordPress Importer Plugin.
These contents are imported after the composer install command.
The WXR files that will be imported for the WordPress installation.
The plugins zip packages that aren't in the WordPress repository.
The themes zip packages that aren't in the WordPress repository. Normally is used as parent of the project theme.