The starting Y-position of the current scroll sprite, left-shifted twice and with the extra bits still added. (Format: --YYYYEE, where Y = Y position and EE = extra bits).
Regularly adjusted inside the scroll sprites. It can be particularly useful for determining what type of scrolling should be applied with this scroll sprite.
This address is used for Layer 1 scrolling.
Also used during the castle destruction cutscenes as a frame number for various animations.