What is a custom resource?

What is a custom resource? service-design.pdf (p.32)

A.
Custom resources are new provisioned items that attach to a service.

B.
Custom resources configure post-provisioning operations that users can perform on provisioned items.

C.
Custom resources are a type of provisioned item mapped to an existing vRealize Orchestrator object type.

D.
Custom resources configure the properties of blueprints using VMware vSphere endpoints.

Explanation:
Reference: https://pubs.vmware.com/vra-62/topic/com.vmware.ICbase/PDF/vrealize-automation-62-advanced-