[FFmpeg-user] Confusion over stream specifier on -map, -codec, and -filter_complex

[gqqnb2005]

Basically, I want to figure out how to specify streams for the option -map,
-codec,  and -filter_complex.

ffmpeg -i INPUT -map 0:1 out.wav

Is 0:1 a stream specifier (
https://ffmpeg.org//ffmpeg.html#Stream-specifiers-1)? Which form is it?

ffmpeg -i INPUT -map 0 -c copy -c:v:1 libx264 -c:a:137 libvorbis OUTPUT

v:1 and a:137 seem to follow the syntax of stream specifier because a and v
are stream_type.

ffmpeg -i in.flac -filter_complex 'acrossover=split=1500[LOW][HIGH]' -map
'[LOW]' low.wav -map '[HIGH]' high.wav

Is '[LOW]' in the syntax of stream specifier?