-map 0:0 -map 0:2 means ‘take the first and third streams from the first input file’.-map 0:1 -map 1:0 means ‘take the second stream from the first input file and the first stream from the second input file’.When no mapping is specified in an ffmpeg command, the default for video files is to take just one video and one audio stream for the output: other stream types, such as timecode or subtitles, will not be copied to the output file by default. If multiple video or audio streams are present, the best quality one is automatically selected by FFmpeg.
To map all streams in the input file to the output file, use -map 0. However, note that not all container formats can include all stream types: for example, .mp4 cannot contain timecode.
To safely process files that may or may not contain given a type of stream, you can add a trailing ? to your map commands: for example, -map 0:a? instead of -map 0:a.
This means that audio streams will be mapped over if they are present in the file—but if the file contains no audio streams, the transcode will precede as usual, minus the audio stream mapping. Without adding the trailing ?, FFmpeg will skip over files which don't contain an audio track.
This is especially recommended when batch processing video files: it ensures that all files in your batch will be transcoded, whether or not they contain audio streams.
+When no mapping is specified in an ffmpeg command, the default for video files is to take just one video and one audio stream for the output: other stream types, such as timecode or subtitles, will not be copied to the output file by default. If multiple video or audio streams are present, the best quality one is automatically selected by FFmpeg.
For more information, check out the FFmpeg wiki Map page, and the official FFmpeg documentation on -map.