This lines says is that VP8 is asigned to payload type 100. It will mean that the value of the Payload Type field of the RTP packets containing VP8 video frames within this session will be 100. This RFC Draft defines the particularites of VP8 PT. Currently there is no MPI codec in WebRTC. Right now VP8 is effectively the MTI codec for video since it is supported in both Chrome and Firefox today, but things may change in the future.