SnapMirror retains the Snapshot copies on the original source volume needed for transfers to destinations further down the line. Snapshot copies that are still needed by a destination are labeled “snapmirror” in the output of the snap list command. SnapMirror deletes the Snapshot copies it no longer needs.
If you remove a destination from the cascade, you use the snapmirror release command from the immediate source to tell SnapMirror to delete the Snapshot copies associated with that destination.