Subscription relationships have a lot of potential flexibility. Some subscriptions can be applied to a physical machine or to a certain number of virtual machines, while others can be applied to a physical host and then inherited by guests.

For subscriptions to be managed effectively, there has to be an internal awareness in the subscription service of the relationships between hosts and guests. This way, a subscription service can properly attach a single physical subscription to a physical host and then apply an included virtual subscription to its guest (for example), rather than consuming two physical subscriptions for each instance.

This association is done by extracting a universally unique identifier for each guest and associating it with its hypervisor. These UUIDs are part of the system facts for each virtual system.

The hypervisor is registered first with the subscription service, and then a related process on the system scans for any guests and submits the discovered UUIDs to the subscription service. This is done through the libvirt process when the virt-who command is run.

There are three factors that must be true for the subscription service to recognize the host/guest association and properly attach subscriptions: