The conferenceID is a unique call identifier to associate all userIDs for a conference. All participants in a common call must have the same conferenceID.
It is important to choose unique a conferenceID for each conference, as you can use the conferenceID to search for a specific conference call. Typically, the reported conferenceID is part of the the URL or SIP/XMPP session ID. For example, in the URL https://www.hangouts.google.com/a342mvn33, the conference ID is a342mvn33.