A Cluster Sound is a Sound to visually mark your signal flow. It's audio transparent, like the Annotation Sound.
When we are constructing complex Sounds we have many prototypes and branches of audio and control. With Cluster Sounds you can mark the start and the end of a process for a better understanding and future reference.
Also they are great for pasting Sounds. (See Wish section)
-- JesusGestoso - 21 Jan 2006