This is a line in the Wave section of the Aud.ini file that sets the buffer size for bouncing tracks. At a value of 0, the bounce buffer is the same size as the Mixing Latency value that you set in
Edit > Preferences > Audio - Driver Settings. If you find that bouncing tracks, especially with certain soft synths, takes a very long time, you can set this value to 100, or some value between 0 and 350 so that the bounce buffer will use a more efficient size for bouncing, which has different requirements from normal playback latency.
Note: on larger projects, setting this variable to a large value can cause out-of-memory errors.