Understanding ffmpeg map option - ffmpeg

Question

I came across the following command in a blog post which creates different quality video streams that are packaged into DASH format via the dash muxer.
ffmpeg -i $VIDEO_IN \
-preset $PRESET_P -keyint_min $GOP_SIZE -g $GOP_SIZE -sc_threshold 0 \
-r $FPS -c:v libx264 -pix_fmt yuv420p -c:a aac -b:a 128k -ac 1 -ar 44100 \
-map v:0 -s:0 $V_SIZE_1 -b:v:0 2M -maxrate:0 2.14M -bufsize:0 3.5M \
-map v:0 -s:1 $V_SIZE_2 -b:v:1 145k -maxrate:1 155k -bufsize:1 220k \
-map v:0 -s:2 $V_SIZE_3 -b:v:2 365k -maxrate:2 390k -bufsize:2 640k \
-map v:0 -s:3 $V_SIZE_4 -b:v:3 730k -maxrate:3 781k -bufsize:3 1278k \
-map v:0 -s:4 $V_SIZE_4 -b:v:4 1.1M -maxrate:4 1.17M -bufsize:4 2M \
-map v:0 -s:5 $V_SIZE_5 -b:v:5 3M -maxrate:5 3.21M -bufsize:5 5.5M \
-map v:0 -s:6 $V_SIZE_5 -b:v:6 4.5M -maxrate:6 4.8M -bufsize:6 8M \
-map v:0 -s:7 $V_SIZE_6 -b:v:7 6M -maxrate:7 6.42M -bufsize:7 11M \
-map v:0 -s:8 $V_SIZE_6 -b:v:8 7.8M -maxrate:8 8.3M -bufsize:8 14M \
-map 0:a \
-init_seg_name init\$RepresentationID\$.\$ext\$ -media_seg_name chunk\$RepresentationID\$-\$Number%05d\$.\$ext\$ \
-use_template 1 -use_timeline 1 \
-seg_duration 4 -adaptation_sets "id=0,streams=v id=1,streams=a" \
-f dash Dash/dash.mpd
The thing I'm struggling to understand is how the maps work. I've read the Map docs but they don't seem to have a comparable example.
Why is -map v:0 used for every video stream?
Why does the first map line operate on stream 0 (e.g. -b:v:0) and the next map line operates on stream 1 (e.g. -b:v:1), etc.
Would really appreciate a breakdown.

Answer 1

Why is -map v:0 used for every video stream?
The correct string should be -map 0:v:0. The file index was omitted and thus implicitly interpreted as file index 0. Such syntax is best avoided.
Why does the first map line operate on stream 0 (e.g. -b:v:0)
Don't think in terms of lines or proximity of options in relation to a map. The 0 in -b:v:0 is part of a stream specifier and targets which output stream to target.
See
What the different between -b:v <target bitrate> and -b <target bitrate> FFmpeg flags?
https://superuser.com/q/1219784/