So, my main interest in using a doormat type mod, is to limit filth spread by transferring it from the pawn, to the carpet. Clean your feet at the door, etc. I'm sure most people have that in mind.

From what I understand based on reddit/wikis/steam comments (could be wrong or outdated), every time a pawn enters a new tile, they first have a 5% chance to transfer a single filth stack to the ground, if the pawn has any. Then they have a 10% chance to try and obtain filth stacks. If they don't have a terrain filth stack yet and are on terrain that generates them, they'll obtain one and can only ever have one of this type. If the tile is dirt/sand, they'll obtain an extra stack specific to that. If the tile they're on already has existing stacks of filth, then they'll pick one of those up, removing one stack from the source. Hence how a pool of 4 blood stacks can slowly end up transferred all over the place. Pawns also have a small chance of creating trash filth stacks on constructed floors, and animals have a chance to create animal filth stacks anytime they move.

Simplified, every time a pawn/animal moves to a new tile, they roll the dice on generating or moving filth.

Tiles can only have 5 stacks of filth, which is what causes it to overflow into nearby tiles when too much piles up in one spot. But even with this limitation, in theory this means that 5 stacks of filth in one tile, will be 5 times less likely to be carried elsewhere when compared to 5 tiles with 1 stack of filth each. Which is pretty good.

Is the auto cleaning thing, really necessary? Up to 5x spread reduction should be really noticeable as long as pawns are cleaning the area around the mats so it can't propagate forever. And ultimately, only replacing filth generating floors with alternatives, can reduce (but not stop) filth generation inside your base, without resorting to auto cleaning mods. Conceptually there may be an issue as well. It's described as "slow" cleaning.. but it would need to be fast enough to prevent pawns from stepping on freshly deposited overflow filth, or it doesn't actually prevent the immediate chance to spread from stepping on said overlfow. So idk ¯\_(ツ)_/¯. Not a huge deal, but I thought I'd share my thoughts and ask about it. Lots of comments regarding their spooky door mat magically cleaning the area heh.