The SpriteSourceIndex attribute specifies the index of the source image to be displayed as this sprite.
This attribute has no default: a value is always required for a descriptor to be usable. The value can be 0: in spritemaps this identifies the field itself as the "sprite" to which any defined behavior will be applied. In sectormaps, the value 1 has a special meaning. For a detailed description of the role of SpriteSourceIndex in spritemaps and sectormaps, see the introduction to
SpriteDescriptor Class.