S-CSCF
Serving Call Session Control Function: the central IMS node that handles SIP registration, session control, service trigger point evaluation, and routing to application servers.
The S-CSCF is the brain of the IMS signalling path. It's where a user is registered, where their service profile (downloaded from the HSS/UDM) lives for the duration of that registration, and where every SIP request to or from that user is evaluated against the initial Filter Criteria — the rules that decide which application servers to invoke and in what order. Call forwarding, call barring, voicemail triggers: those decisions are made here by routing the SIP dialogue out to the relevant TAS and back.
Because it holds registration state and the service logic, the S-CSCF is the stateful heart of the system, unlike the mostly stateless I-CSCF. It authenticates the registration (the SIP-layer AKA challenge happens through it), keeps the registration alive via the expiry timer, and sits in the signalling path for the whole session so it can keep applying triggers. When people sketch "how a VoLTE/VoNR call is set up," the S-CSCF is the node doing the orchestration in the middle.
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