WebRTC - SDP has more m= line - webrtc

Question

In my app where one peer (A) has one media stream, and other side (B) has two media streams.
I am not understanding why there are more m= line in offer from B to A:
1) The offer from A to B is as follows:
v=0
o=- 298896314257884346 2 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE 0 1
a=msid-semantic: WMS BVmismsFWNYofSpFcVAnbDtyGxd1hzVGcOXe
m=audio 9 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126
c=IN IP4 0.0.0.0
...
a=mid:0
...
a=sendrecv
a=msid:BVmismsFWNYofSpFcVAnbDtyGxd1hzVGcOXe 982eedc1-919b-450f-80e3-2fb173cbc5ae'
...
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 123 127 122 125 107 108 109 124
c=IN IP4 0.0.0.0
...
a=mid:1
...
a=sendrecv
a=msid:BVmismsFWNYofSpFcVAnbDtyGxd1hzVGcOXe 4112fe2c-24b3-4135-9ad6-7e2f38a6692f
2) The offer from B to A is as follows:
v=0
o=- 6641223000983904101 3 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE 0 1 2 3 4
a=msid-semantic: WMS
m=audio 54011 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126
...
a=mid:0
...
a=sendrecv
a=msid:- 4fb1ecb1-86e7-4529-a5b6-1dddd6f77ce0
...
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 123 127 122 125 107 108 109 124
...
a=mid:1
...
a=sendrecv
a=msid:- b3b97f66-8a7a-4aaf-857b-e011648a4a64
...
m=audio 9 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126
...
a=mid:2
...
a=sendrecv
a=msid:LukrKSP3g6xYlKFbI9DhHXmE22ZEwNz3Shpo 06363625-089b-49f8-ab23-ec04ac0499c0
...
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 123 127 122 125 107 108 109 124
...
a=mid:3
...
a=sendrecv
a=msid:LukrKSP3g6xYlKFbI9DhHXmE22ZEwNz3Shpo 108478fa-20a9-4393-aaf2-3d27ac641258
...
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 123 127 122 125 107 108 109 124
...
a=mid:4
...
a=sendrecv
a=msid:zCWSiL1IMAhK6skk975lr4IIPf4LBoh2chQI 4600414c-8faf-451c-b6d4-7d8310ab63f8
In 2nd case the m= line is 5 which is in (BUNDLE 0 1 2 3 4)
Why there are 5 lines when we have only two video streams?

Answer 1

session description includes information about the kind of media being sent, its format, the transfer protocol being used, the endpoint's IP address and port, and other information needed to describe a media transfer endpoint.
Since B has two media streams SDP will contain 4 m lines (2 per stream).
In Unified Plan every single media track is assigned to a separate “m=” section. If you use multiple media tracks, multiple “m=” sections are created.
Chrome is migrating to unified plan from plan-b
If you want more details about unified plan you can see this blog - https://www.callstats.io/blog/what-is-unified-plan-and-how-will-it-affect-your-webrtc-development