3GPP 24.501 v20.0.0 — the document's own text
6.1.3.2.6 PDU SESSION MODIFICATION PENDING
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The UE has initiated a PDU session modification procedure towards the network and is waiting for a response from the network.