A cluster has many points bedded, but not necessarily fixed, into a base. The crystals may be small or large. Clusters radiate energy out to the surrounding environment and can also absorb detrimental energy. They can be programmed and left in place to do their work. They are especially useful for cleansing a room or other crystals, in which case the crystal should be left overnight on the clusters.
This information is from the book THE CRYSTAL BIBLE 1 By Judy Hall.