diff --git a/pkg/exposer/csi_snapshot.go b/pkg/exposer/csi_snapshot.go
index 535268a3d..247ae9003 100644
--- a/pkg/exposer/csi_snapshot.go
+++ b/pkg/exposer/csi_snapshot.go
@@ -585,6 +585,14 @@ func (e *csiSnapshotExposer) createBackupVSC(ctx context.Context, ownerObject co
 			// of forcing Delete, otherwise a user who configured Retain on the
 			// VolumeSnapshotClass still loses the snapshot when the backup VSC
 			// is cleaned up.
+			//
+			// For Case 2 storages per the design (design/block-data-mover/block-data-mover.md,
+			// e.g. Ceph RBD), inheriting Retain is not just an option but a requirement for
+			// incrementals to work at all: rbd snap diff needs the base and target snapshots
+			// in the same clone chain, so Delete destroys the base as soon as this backup
+			// completes. The next incremental's delta query then fails and degrades to an
+			// allocated-blocks backup (see the CBT tier ladder) or, without that fix, a full
+			// whole-device transfer.
 			DeletionPolicy:          snapshotVSC.Spec.DeletionPolicy,
 			Driver:                  snapshotVSC.Spec.Driver,
 			VolumeSnapshotClassName: snapshotVSC.Spec.VolumeSnapshotClassName,