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Definitionally, a Subscription requires the a SubscriptionTopic - without a resource describing the change of interest, a Subscription has no meaning and will not trigger any client notifications.

While active, a Subscription relies on both Bundle and Parameters for sending notifications.

When using the Subscription resource, the FHIR server combines the roles of publisher and information distributer. Some arrangements of the ‘publish and subscribe’ pattern describe separate agents for the two roles. This specification makes no recommendations towards the internal architecture of server implementations.

into the subscription bullet and as sub-bullets