The default stream selection will choose streams based upon specific criteria.For example, if the input has 3 video streams it will only choose 1. Only 1 stream per type will be selected.Default stream selection will not automatically choose all of the streams.If you do not use the -map option then the default stream selection behavior will automatically choose streams. -map 0 -map -0:s Will select all streams from input index #0 (the 1st input) except subtitles.-map 3:s:4 From input index #3 (the 4th input) select subtitle stream index #4 (the fifth subtitle stream).-map 1:a From input index #1 (the 2nd input) select all audio streams.-map 0 From input index #0 (the 1st input) select all streams.map 1:a:2 refers to 'second input:audio only:third audio stream'. The bottom command includes stream specifiers ( v for video, a for audio) to limit the streams to a specific type.map 1:3 refers to 'second input:fourth stream'.

This is an absolute method of mapping and will choose a specific stream regardless of type. The top command does not use any stream specifiers.The commands do the same thing, but use different syntax to map streams: The commands in the diagram above will select the video from input0.mp4 and the 3rd audio stream from input1.mkv to output.mp4. The -map option can also be used to exclude specific streams with negative mapping. The -map option is used to choose which streams from the input(s) should be included in the output(s).

Ffmpeg map plugin