what is Background transcoding x264 preset setting under transcoder used for like ultra fast and super fast setting?
This is used for transcodes which are performed as preparation to sync content to mobile devices or into cloud storage.
The optimize feature also uses background transcodes.
so my question is should I use ultra fast or super fast setting on it
It depends on what you (or more: your server’s cpu) can bear. 
The ‘slower’ the preset, the smaller is the file size of the transcoded version. (for sync transcodes)
If the target bitrate is fixed (like in ‘optimize’ operations) the transcoder can achieve a higher video quality at the slower presets.
