Simple definition
Immutable backups prevent stored restore points from being changed or deleted during a defined protection period.
Purpose and use
Immutability protects backup versions against selected modifications or deletion for a defined period.
How does it work?
It can rely on a hardened repository or object locking; guarantees depend on the chosen mode, duration and effective permissions.
Example and verification
Inspect protection on the actual written objects or restore points and validate access through an appropriate test scenario.
Points to watch
Immutability does not replace backing up healthy data or retaining decryption keys. Some modes also allow specifically privileged accounts to bypass retention.
In short
Assess the mechanism and protected scope rather than relying only on an enabled console option.