Clipped from "pan, tilt, roll" in the manual...
Angles have a special meaning with sprites. If all angles are at zero, the sprite will stand upright
and horizontally face the camera. If its pan or tilt angle is nonzero, the sprite is oriented in
world space according to its angles. If pan and tilt both are 0, but the roll angle is nonzero, the
sprite is always perpendicular to the camera. This is useful for spherical objects, like fireballs or
explosions. For an angle to be nonzero it's sufficient to set it at a small amount, like 0.01.