The PCRF can request IP-CAN session termination on the node via two messages:
a RAR directive with the Session-Release-Cause AVP to the node
ASR
Upon the arrival of either of those messages, the node starts the IP-CAN session termination procedure (CCR-T with a corresponding Termination-Cause AVP is sent to the PCRF). This is described in the 3GPP 29.212 document, §4.5.9.
For a list of the supported Termination-Cause AVP values on the node, see the 7750 SR and VSR Gx AVPs Reference Guide.