SpriteBatch#hash

The hash array is an array belonging to this Group into which you can add any of its children via Group.addToHash and Group.removeFromHash.

Only children of this Group can be added to and removed from the hash.

This hash is used automatically by Phaser Arcade Physics in order to perform non z-index based destructive sorting.
However if you don't use Arcade Physics, or this isn't a physics enabled Group, then you can use the hash to perform your own
sorting and filtering of Group children without touching their z-index (and therefore display draw order)