At start, this role is effectively an ordinary Townie. However, whenever the first power role dies (i.e. Doctor, Cop, Vigilante, etc.), the Universal back-up inherits that power role and can use it themselves. Thus, Universal Backup is a Backup that is not specific to any particular role.
Universal Backups are not necessarily self-aware; they may learn they are a Universal Backup when they inherit an ability.
Universal Backups are generally Town-aligned, as in Mafia hands they make games become very swingy depending on what role they pick up.
The term "power role" is somewhat ambiguous - generally anything that is not truly Vanilla is considered a "power role", including roles like Miller. It is ultimately up to the moderator's discretion which abilities are inheritable, though it should be planned in advance.
Universal Backup (if self-aware) is considered Normal on mafiascum.net. A Universal Backup can be of any alignment, but it should only activate upon the first death of a power role of their own alignment. A power role in this case is any role that is not Vanilla (so not a Vanilla Townie or a Mafia Goon). Universal Backups inherit any and all modifiers, including the remaining shots of an X-Shot role.
Use and Power
Universal Backup is considered swingier than single-role backups like Deputy or Nurse, but depending on the setup, it can be more powerful simply because it is more likely to back up an ability than a single-role backup.
Thus, its power varies. The game should be acceptably balanced in the event that the Universal Backup inherits any ability feasible in the setup.
If the game has multiple killing roles with potential for multiple power role deaths, then the mod must resolve which killer strikes first.