The rollback procedure reverts all of the system software and all of the metadata settings of the system back to their state just prior to applying an update. This feature is implemented by taking a snapshot of various aspects of the system before the new update is applied, and rolling back this snapshot to implement the rollback. The implications of rollback are as follows:
Any appliance configuration changes are reverted and lost. For example, assume you are running version V, and then you upgrade to V+1, and then you change your DNS server. If you execute a rollback, then your DNS server setting modification is effectively undone and removed from the system permanently.
Conversely, any changes made to user data are not reverted: if you upgrade from V to V+1, and clients then create directories or modify shares in any way, those changes still exist after the rollback (as you would expect).
If the appliance is running version V, and has previous rollback targets V-1 and V-2, and you revert all the way to version V-2 (thereby "skipping" V-1), then you not only are removing the system software settings and system software for V, but also for V-1. That is, after a rollback to V-2, it is as if updates V-1 and V never happened. However, the software upload images for V-1 and V are saved on the system, and you can apply them again after the rollback, if you wish, by re-executing the update.
If after applying an update, the system is back up and running, you can use either the BUI or the CLI to initiate a rollback to one of two previously applied updates. If the system is not able to run at all after an update, then use the fail-safe rollback procedure.