Custom FFmpeg Export Options
From Audacity Development Manual
'Peter 12Aug15: ToDo-1 As far as I can see this has never been documented.
I'm pretty stuck as how to proceed further with this - it's way beyond my envelope of knowledge and experience ...
The Custom FFmpeg format options dialog appears in response to the button in the Export Audio or Export Multiple dialog Options pane when you have chosen Custom FFmpeg Export in the Save as type dropdown.
