BVR supports basichttp I/O events as documented here https://www.w3.org/TR/ccxml/#basichttpio
The session.fullid contains the full CCXML session ID. This is used for incoming basichttp requests and is the same ID reported from sessions created using createsession. The basichttp requests are sent to the BRM which picks the correct BVR from the full CCXML session ID and forwards the requests to that BVR's CCXML session.