Next we have the ssrc related lines. SSRC stands for Synchronization Source and it is an unique identifier of the RTP media stream source. In this we have many sources. Roughly speaking, these lines establish a relationship between the SSRC flow identifier - an older RTP concept – to a WebRTC Media Stream – a newer WebRTC concept.
The label attribute carries a pointer to a RTP media stream in the context of an arbitrary network application that uses SDP. This label can be used to refer to each particular media stream in its context.