Slot of platform sprite that Yoshi is on. Does not apply to every platform sprite, for example it does for Pinwheels, Chomp Rock, Poochy, Flatbed Ferries, etc, but not for Hint Blocks, Donut Lifts, etc.
Where the water line starts for sprite physics
Only used for BG3 tilesets $13 (value $07B0) and $1D ($0700)
Value is left at $4000 when disabled
This doesn't affect regular water objects, only hardcoded water for certain BG3 tilesets.
ss = vertical screen number
yy = Y-position of water line
Screen ID table: maps raw screen # ($00-$7F) to screen ID ($00-$3F). The sign bit on indicates "empty" screen (has no object at all), or camera is unable to scroll into the screen due extended object $FE, or tiles are erased in that screen by extended object $FF.