3GPP 24.501 v20.0.0 — the document's own text
8.3.12.3 Extended protocol configuration options
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
This IE is included in the message when the UE needs to transmit (protocol) data (e.g. configuration parameters, error codes or messages/events) to the network.