One of main advantages of Kubernetes API is flexibility; users can add a custom resource to the Kubernetes cluster, and Kubernetes apiserver manages defined custom resources like standard resources (e.g. ReplicaSet, etc). Main introduction in Kubernetes document is in [here]. A major confusing point comes from ambiguous distinction between Custom Resource Definition (CRD) and Aggregated APIserver (AA). Even the document explains some differences of two types of implementation, it is not clear...
