[FFmpeg-user] What does -write_xing 0 do?

[MediaMouth]

I've been testing the option -write_xing 0 so that Apple Music properly displays the durations on ffmpeg-generated mp3 files.

I'm aware of the option from this 7-year-old post: https://trac.ffmpeg.org/ticket/2697 <https://trac.ffmpeg.org/ticket/2697> which describes it as a solution to QuickTime displaying TRTs incorrectly.

The problem mp3 files I've been testing actually show proper TRTs in QuickTime, but not in Apple Music and ExifTool.

The -write_xing 0 option seems to be fixing it.  I'm curious what it's actually doing.
What was the issue to begin with / what problem is it solving?
Is it doing other things to the converted file that might be worth knowing about?  Any down sides to using it?