The appliance deletes update packages after installing an update. Rolling back an update removes the last intrusion prevention update installed on the appliance. When a new update is available, the appliance applies any missing updates, including updates not applied to the appliance. If you decide to roll back from the new update, the appliance rolls back to the version last applied.
If you install version 1.1 but do not install version 1.2, when you install version 1.3, the appliance adds version 1.2 with version 1.3. If you roll back from version 1.3, the appliance rolls back to version 1.1. It does not roll back to version 1.2 because you did not originally apply that update version.