[FFmpeg-user] I'm lost with the options to set a default subtitle

[pehache]

Say I have an input file with several subtitles, one of them being 
flagged "default" (say the 0:s:1 stream, that is the 2nd subtitle 
stream). I am copying all of them: "-map 0:s -c:s copy"

Empirically I have found that:

* no option
-----> the default flag is unmodified

* "-disposition:s:2 default"
-----> the default flag is unmodified

* "-disposition:s -default"
-----> the default flag is set on the s:0 stream

* "-disposition:s -default -disposition:s:2 default"
-----> the default flag is set on the s:2 stream

* "-disposition:s -default -disposition:s:2 default"
-----> the default flag is set on the s:2 stream

* "default_mode passthrough"
-----> the default flag is unmodified

* "default_mode infer"
-----> the default flag is unmodified

* "default_mode infer_no_subs"
-----> the default flag is unmodified

* "default_mode infer -disposition:s -default"
-----> the default flag is set on the s:0 stream

* "default_mode infer_no_subs -disposition:s -default"
-----> the default flag is removed from all s: streams

* "default_mode passthrough -disposition:s -default"
-----> the default flag is removed from all s: streams

So, I know what to do depending on what I want on the output, but I do 
not understand at all how these different options interact...

Any clue appreciated!