Plan B is an IETF draft proposal for signaling multiple media sources in SDP.

Signaling multiple media sources is needed to be able to send a video along with screen sharing, multiple camera sources or routing multiple streams from an SFU to the users.

In Plan B, an m= line is an “envelope” that includes multiple media sources per one defined transport.

The mechanism chosen for the official WebRTC specification isn’t Plan B but rather Unified Plan.

Plan B has been deprecated and removed from browsers.