3GPP 24.501 v20.0.0 — the document's own text
8.2.10.6 DNN
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The UE may include this IE when the Request type IE is set to "initial request", "existing PDU session" or "MA PDU request", the Payload container type IE is set to "N1 SM information" and the UE is not registered for onboarding services in SNPN.