A service is a group of catalog items that you want included in the service catalog. You can entitle the service, which entitles business group users to all the associated catalog items, and you can apply an approval policy to the service.
A service operates as a dynamic group of catalog items. If you entitle a service, all the catalog items associated with the service are available in the service catalog to the specified users, and any catalog items that you add or remove from a service affect the service catalog.
As you create the service, you can use it as a service category so that you can assemble service offerings for your service catalog users. For example, a Windows desktop service that includes Windows 7, 8, and 10 operating system catalog items, or a Linux service that includes CentOS and RHEL operating system items.