The bucket, is basically a place that stores a bunch of "Background files" you want played repeatedly...
Lets say you want to focus on 2 or 3 watermark files rather than your entire list of watermarks, you want them played softly underneath all your songs.
Well, that's what the bucket-system does. The bucket is at the bottom of the window.
The 'bucket' automatically collects played watermarks and tone-test files that.