Thank you for the quote, but IMO B.1 isn't relevant here. We're not discussing whether 2J18 is a building hex: it's a rubble hex--I think most posters agree on that. The issue is whether the building depiction that juts a little bit into 2J18 blocks LOS.
In normal ASL play, if the depiction of the still-standing portion of the building in 2I18 only reached three-quarters of the way across hex 2I18, then it wouldn't block high-elevation LOS in that last quarter of 2I18 where the building is drawn as being collapsed.
So by a parallel argument, I would say that the depiction of the still-standing building that juts partway into 2J18 is meant to block LOS there.
I.e., it's the building depiction itself that blocks LOS.
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A6.1 CHECKING LOS: The ability of a unit to “see” and “fire directly” at another unit in a different hex is dependent on having a LOS between those units. Whether a unit has a LOS to a given hex is usually determined by stretching a sewing thread taut between the center of the firing hex and the center of the target hex. If the thread does not cross a terrain depiction capable of obstructing the LOS between the target and firing hexes (with the obstruction visible on both sides of the thread) [EXC: Inherent Terrain hexes (B.6) block or hinder LOS if traced exactly along their hexside even though the terrain depiction is not visible on both sides of the thread], there is a clear LOS between the two hexes. If players still cannot agree whether a LOS is blocked or not, the matter is resolved by a dr: 1-3 LOS is not blocked; 4-6 it is blocked. It is usually (but not always; see B.6 & B9.1) the terrain depiction which can potentially block or hinder LOS—not the hex containing that terrain type.
A6.2 OBSTACLES: Each terrain type [EXC: Depressions 6.3] is defined as to whether or not it presents an obstacle or Hindrance to LOS and, if an obstacle, the height of that obstacle. LOS extends into or out of obstacles, but not through them into hexes beyond the obstacle except in certain cases wherein the target/firer are at an elevation ≥ the height of the obstacle or are adjacent to a hexside obstacle. The terrain in a firer’s hex never blocks LOS traced from its hex center dot, although it may block LOS traced from a vertex of its hex across the interior of its own hex.