Sprite Extension Word

Hi all,

Can somebody direct me to some information about the sprite extension word/s which live/s at offset &C in a spritefile?

All it says in my copies of the PRM is it/they are are “usually null” but that isn’t so, it which changes depending on the sprite’s characteristics. Changing it to zero corrupts the sprite and I can’t find any pattern to the numbers.

Thanks :)
Chris