##
Background Rendering section

Use background rendering
  checking this box, enables automatic background rendering. This works in conjunction with the interactive function Settings menu → Toggle background rendering which sets the point where background rendering starts up to the position of the insertion point.
Frames per background rendering job
  his only works if a render farm is being used; otherwise, background rendering creates a single job for the entire timeline. The number of frames specified here is scaled to the relative CPU speed of rendering nodes and used in a single render farm job.
Frames to preroll background
  the number of frames to render ahead of each background rendering job. Background rendering is degraded when preroll is used since the jobs are small. When using background rendering, this number is ideally 0. Some effects may require 3 frames of preroll.
Output for background rendering
  background rendering generates a sequence of image files in a certain directory. This parameter determines the filename prefix of the image files. It should be on a disk, accessible to every node in the render farm by the same path.
File format
  the file format for background rendering has to be a sequence of images. The format of the image sequences determines the quality and speed of playback. JPEG is a good choice usually.
Video wrench
  this has the single option of use alpha. It is by default unchecked.