Both the "Custom FFmpeg" and "Custom AviSynth-script" are available for you to do things within FFAStrans that are not natively featured. And so they require a generic knowledge of FFmpeg and AviSynth to make it work. Official documentation can be found here:
http://www.ffmpeg.org/ffmpeg-all.html
http://avisynth.nl/index.php/Main_Page
Also, there are plenty of resource pages on the internet for both these two systems.
I'm keen to help everyone to get the most out of FFAStrans but I don't have the resources do generic FFmpeg or AviSynth support. So here are the guidelines:
All requests/questions about either systems should include a working example of what you want to accomplish using FFmpeg and/or AviSynth. It is then much easier to find a way to incorporate it into FFAStrans. That is not to say I wont help you but if you fail to follow this guideline you are not guaranteed my attention. Hope this is respected and understandable to all on this forum.
Thanks
-steinar