NSSI
Network Slice Subnet Instance: a component of an NSI representing a specific network domain (RAN, transport, core), which can be shared across multiple network slice instances.
An NSSI is a per-domain piece of a slice. Where an NSI is the whole end-to-end slice instance, an NSSI is the chunk of it living in a single domain — a RAN slice subnet, a transport slice subnet, a core slice subnet — each managed by the management function responsible for that domain.
What makes the concept useful is sharing. An NSSI can be dedicated to one slice or shared across several, so a single core slice subnet might serve multiple NSIs that each have their own distinct RAN subnet. That decomposition is also how slice management is divided up: a domain manager handles its own NSSIs and exposes them upward, and the cross-domain orchestrator stitches NSSIs together into complete NSIs. So the layering goes slice template → NSI (end-to-end) → NSSIs (per-domain components), and most of the reuse and isolation decisions in a slicing deployment happen at the NSSI level.
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