From the Kingdom Hearts Wiki: A world of information not accessible by Gummiship
Cluster Cannonade is a max-level Shotlock associated with the Shooting Star Keyblade, activated by achieving 18 locks on a group of enemies. When activated, Sora Formchanges the Keyblade into Magic Launcher and fires a rocket that splits into smaller particles. Visually, Sora's clothes change into Element Form when using this Shotlock.
Learning Cluster Cannonade
Kingdom Hearts III