Redundancy is a form of resilience that ensures system availability in the event of component failure.
The level of resilience is referred to as active/passive or standby as backup components do not actively participate within the system during normal operation.
1+1 redundancy typically offers the advantage of additional failover transparency in the event of component failure.
The level of resilience is referred to as active/active or hot as backup components actively participate with the system during normal operation.
The reason for this, in the case of UPSes, is that a common failure mode is component's end-of-life failure, and if UPSes are equally used, then they are highly likely to fail within a short space of time of each other when toward the end of service life.