The Synology D4NESO-2666-4G is a 4GB DDR4 2666 MHz non-ECC unbuffered SO-DIMM memory module designed for compatible Synology NAS systems. It provides a reliable memory expansion option for supported DiskStation and RackStation models, helping improve system responsiveness and support multi-user workloads. With a 260-pin SO-DIMM form factor and 1.2V operating voltage, it is designed and tested by Synology for compatibility with supported NAS environments.