This is an external battery pack for APC's Smart-UPS RT series of rack-mount uninterruptible power supplies, designed to extend the runtime a connected UPS can provide during a power outage beyond what its internal batteries alone deliver. It runs at the 192V bus voltage used by RT-series units and installs as an additional rack-mount module alongside the UPS itself ('RM' denotes rack-mount). This is an accessory/expansion component, not a standalone UPS — it needs to be paired with a compatible APC Smart-UPS RT unit to be useful — and is bought by businesses that need longer battery backup time for servers or critical equipment than the base UPS configuration provides.