Detailed explanation of network slicing NSSAI in 5GC
For 5G wireless networks, network slicing is mainly reflected in admission control, network selection, and resource separation. The standard is mainly identified by S-NSSAI (Single Network Slice Selection Assistance Information) parameters. The definition of this parameter in wireless is as follows: 
SST indicates the slice and service type of S-NSSAI, and sst-SD is the composition and slice component of S-NSSAI parameter slice, service type.
NSSAI is a collection of S-NSSAI. NSSAI can be a configured NSSAI, a requested NSSAI or an allowed NSSAI (Configured NSSAI, a Requested NSSAI or an Allowed NSSAI). In the signaling message between the UE and the network, there can be up to 8 NSSAIs allowed and requested.
According to the operation or deployment needs of the operator, one network slice instance can be associated with one or more s-NSSAIs, and one s-NSSAI can be associated with one or more network slice instances. Multiple network slice instances associated with the same S-NSSAI can be deployed in the same or different tracking areas. When multiple network slice instances associated with the same S-NSSAI are deployed in the same TA, the AMF instance serving the UE can logically belong to (that is, jointly belong to) multiple network slices associated with the S-NSSAI Instance.
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two good articles Reference: Network-NSSAI slice of S , 5G and transmission defined in NSSAI