A cloud service is a modeling Element that is a type of Infrastructure. These can be created using the Add Wizards. There are a couple of key Elements that need to be created for modelling purposes. They represent common parts of a hybrid or pure cloud environment.
A Cloud Datacentre is a logical Element that is the equivalent of an on-premises datacentre. Other cloud services can be dependent on a Cloud Datacentre such as migrated servers.
Every Cloud Service has to live in a "tenant". A Tenant is a presence in Office365, AWS or Microsoft Azure. Cloud Datacentres should be marked as dependent on Tenant Administration.