Is the map tier bonus from voidstones additive with the map bonuses from the tree?

I keep seeing people with conflicting opinions on this and I think that the fact that people think that they are multiplicative is just misinformation caused by the fact that the game doesn't add those numbers together within the map mods list. The game almost never takes mods from the atlas passive tree and puts them in the map mods list, with notable exceptions being the pack size wheel which actually shows up within a map.

I wanted to make this thread so that we finally have a source on this that we can link to whenever someone asks this question. So, to reiterate the question:

Is the map tier bonus from voidstones additive with the Shaping nodes and other small chance for map being of a higher tier nodes?

Additionally, beyond the question of voidstones being additive (or not) with the atlas passive tree nodes, it would be useful to understand the interaction when exceeding 100% chance. For instance, in Ruthless, with 4 voidstones (25% each) and a number of the small atlas passives at the top of the tree (2% each).

To my understanding, if with 4 voidstones our full atlas only contains T16 maps, it's impossible to drop a map of T15 or below.
Using simple logic, adding more % from the atlas passives to the already 100% chance to drop T16 maps, is just nonsense, because you can't drop T15 or below maps.
And I think they created it for a reason, that in the end you can only drop T16 maps. Only bottom half of the atlas tree contains these passives anyway, so from all directions I can notice signs of having these nodes allocated with 4 voidstones already enabled is just a waste of points and no benefit for either more maps or even higher chance of T17 maps, because you're only dropping T16 maps anyway, from which T17 can emerge.

If map drop equity is still a thing (https://www.pathofexile.com/forum/view-thread/3130696, near the bottom), which is probably is, then the map tier bonus stacks additively. Back when we had watchstones it was explicitly stated that over 100% chance to drop 1 tier higher starts giving chance to drop 2 tiers higher, and that is also probably still true.


So, an oversimplified analysis of all this shows that the difference between 200% chance to drop maps 1 tier higher (ie, always dropping 2 tiers higher) and 0% would be getting 9x as many t16 maps.

It's not actually 9x as many maps of course, since when you're running t16s, many map drops will already be t16 regardless of your watchstones/atlas passives. It won't "try" to give you a t17, fail, and give you a t16 + map equity. It just gives you a t16.

With 4 watchstones, only map drops that would've been t14 or lower benefit from atlas passives that boost map tier drop chance. That's actually not very many of the drops you get from t16 maps. So, while those atlas passives do in effect give you more t16 map drops, it's not by very much.

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The game almost never takes mods from the atlas passive tree and puts them in the map mods list, with notable exceptions being the pack size wheel which actually shows up within a map.

As a side note, the list of mods on the right side of the screen doesn't show ALL the mods affecting an area. So technically, all the atlas passives are in fact mods on the map area, it's just that they're all hidden so you don't get spammed with mods.

You only see atlas passive tree mods on that list if you have another source of that same mod that wouldn't be hidden. Item quantity/rarity is an obvious one, as you mentioned. It's also true for mods granted by scarabs or map device crafts. IE, "your maps contain an additional harbinger" from the tree won't normally show, but if you add a harby scarab then the number will be 1 higher than what the scarab adds, since the atlas passive is also included.