SD

SD is the optional second half of an S-NSSAI, a 24-bit field whose only job is to tell apart slices that share the same SST. Without it, SST=1 is just "some eMBB slice." Add SD=A1B2C3 and you've named a specific eMBB slice — which is exactly how an operator carves out per-tenant or per-service slices on the same slice type.

The usual real-world use is multi-tenancy: three enterprise customers each get a private slice that's all SST=1 (broadband), distinguished purely by their SD value. Because SD is operator-defined and only meaningful in context, the same SD can mean different things in different PLMNs — which is why roaming relies on mapped S-NSSAIs to reconcile slice identities across networks. When SD is absent, the S-NSSAI is simply identified by its SST alone.