From the docs (for anyone else reading this)
Local Collision Handling
Local collision handling is best utilized in a one-to-many collision scenario, for example one player object which may collide with multiple enemies, power-ups, etc. For local collision handling, each collision event includes a self table ID, representing the object itself, and event.other which contains the table ID of the other Corona display object involved in the collision.
http://docs.coronalabs.com/guide/physics/collisionDetection/index.html#handling