NSI
Network Slice Instance: a deployed instance of a network slice comprising a set of NF instances and the required resources configured to meet specific service requirements.
If a network slice is the design, the NSI is the running thing. It's an actual deployed instance — a concrete set of NF instances plus the compute, transport, and radio resources, configured and wired together to meet one slice's requirements. You can have several NSIs derived from the same slice blueprint, for example one per region or one per major customer.
The relationship that usually clears things up: an NSI is composed of one or more NSSIs (Network Slice Subnet Instances), each covering a domain like RAN, transport, or core. Subnets can be shared — two NSIs might reuse the same core NSSI while having different RAN behaviour. Managing the NSI's lifecycle (create, activate, scale, modify, retire) is what slice orchestration and the management plane spend their time on. The distinction from "network slice" is the same as class versus object: the slice is the template, the NSI is the instantiation.
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