UDR
Unified Data Repository: the 5GC database that stores subscription data, policy data, structured data for exposure, and application data accessed by UDM, PCF, and NEF.
Underneath the stateless control-plane functions there has to be something that actually keeps the data, and that's the UDR. It's the converged store behind several front ends: subscription data read by the UDM, policy data read and written by the PCF, structured data the NEF exposes externally, and application data. Each consumer reaches it through a defined data-management service rather than poking at raw tables.
The point of pulling storage out into its own function is consistency and scale. Multiple UDM, PCF, and NEF instances can share one logical UDR, so a policy or subscription change made once is seen everywhere, and you scale stateless compute separately from the data tier. In practice it's frequently deployed as part of a larger UDM/UDR/UDSF data-layer product. Note it's distinct from the UDSF, which stores transient per-session state rather than this kind of durable subscription and policy data.
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