The Synology RX6025sas is a high-density storage expansion unit — an enclosure of additional drive bays that connects to a compatible Synology server (the listing references Synology's HD6500 series) to extend total storage capacity beyond what the base unit's own bays allow. Expansion units like this are used when an organisation's storage needs outgrow their primary NAS/SAN chassis, letting IT scale capacity without replacing the whole system or migrating data to new hardware. This is enterprise storage infrastructure aimed at businesses running Synology's higher-end server platforms, not a standalone consumer NAS.