Source epoch is at/after the anchor with retained state present: replayable.
Source epoch is older than the retained anchor: drop as BeyondAnchor.
Inside the rollback horizon but required retained state was lost: → Unrecoverable.
Parent commit has not arrived yet: wait for it (a transport gap, not loss).
At/after the anchor but forks from outside the rollback horizon: not selectable.
The outcome of judging a late commit by its source epoch.