[VoiceOps] RTP End?

We are using a tool called Empirix Hammer to analyze and report on our network traffic. This tool as uncovered something that it calls "RTP End" and I'm trying to get a better definition of what it means. I haven't been able to find anything online, including RFCs etc. Basically when the Hammer see this special packet, it is when a phone or media gateway is tearing down a call. Logically speaking it is identifying the end of a media stream, but I'm looking for a technical definition. Can anyone provide any insight? Thanks, -Clint