This is the high byte of the Map16 tile (actual tile number, not "acts like" setting) that a sprite is touching horizontally. The low byte is at $1860.
This, $185F, $1860, and $18D7 are set after $019138, the subroutine that allows the sprite to interact with objects, is called. (This includes $01802A, the routine that updates sprite position with gravity, which calls $019138.)