Used by the routines at overlap_test_obj, is_on_or_near_obj, dX_obj_loop, dY_obj_loop and dZ_obj_hit_test.

Not centred like X and Y: Z is an object's base and +$06 its whole height. So the test takes the gap between the two bases and compares it with the height of whichever object is lower. The player's top half has a height of 0, which is why it is kept out of the scans while the legs move (player_move_and_draw): the legs, 23 high, would find it inside them.