Currently, the following document instructs us to set a tag on the `releaseImage` line of `ClusterImageSet` resources for disconnected envronments:

https://docs.redhat.com/en/documentation/red_hat_advanced_cluster_management_for_kubernetes/2.11/html/clusters/cluster_mce_overview#release-images-disconnected

apiVersion: hive.openshift.io/v1
kind: ClusterImageSet
metadata:
  labels:
    channel: fast
  name: img4.14.8-x86-64-appsub
spec:
  releaseImage: IMAGE_REGISTRY_IPADDRESS_or_DNSNAME/REPO_PATH/ocp-release:4.14.8-x86_64

However, according to the following changes, 

• https://issues.redhat.com/browse/ACM-11207
• https://issues.redhat.com/browse/MGMT-17194
• https://github.com/openshift/assisted-service/pull/6260

when we use mirror registries, upcoming future releases will only accept a digest, not a tag.

I think we should state that we must use a digest instead of a tag when mirror registries are used in the document.
And the example setting should be like the following example:

apiVersion: hive.openshift.io/v1
kind: ClusterImageSet
metadata:
  labels:
    channel: fast
  name: img4.14.8-x86-64-appsub
spec:
  releaseImage: IMAGE_REGISTRY_IPADDRESS_or_DNSNAME/REPO_PATH/ocp-release@sha256:073a4e46289be25e2a05f5264c8f1d697410db66b960c9ceeddebd1c61e58717

A digest corresponding to a tag can be found by the following document:

1.8.12.3. Extracting the OpenShift Container Platform release image digest
https://docs.redhat.com/en/documentation/red_hat_advanced_cluster_management_for_kubernetes/2.11/html/clusters/cluster_mce_overview#configure-hosted-disconnected-digest-image