What, exactly, doesn't stack when stacking rats?

[VoodistMonk]

I am curious about the last line of this feat... not the Special part, but this:
"This ability does not stack with similar abilities providing a creature benefits for being in another creature’s space, such as a mouserACG swashbuckler’s underfoot attack deed."

But, Mouser does not actually have a deed named "underfoot attack"...
"Underfoot Assault (Ex): At 1st level, if a foe whose size is larger than the mouser’s is adjacent to her and misses her with a melee attack, the mouser can as an immediate action spend 1 panache point to move 5 feet into an area of the attacker’s space. This movement does not count against the mouser’s movement the next round, and it doesn’t provoke attacks of opportunity. While the mouser is within a foe’s space, she is considered to occupy her square within that foe’s space.

While the mouser is within her foe’s space, the foe takes a –4 penalty on all attack rolls and combat maneuver checks not made against the mouser, and all of the mouser’s allies that are adjacent to both the foe and the mouser are considered to be flanking the foe. The mouser is considered to be flanking the foe whose space she is within if she is adjacent to an ally who is also adjacent to the foe. The mouser can move within her foe’s space and leave the foe’s space unhindered and without provoking attacks of opportunity, but if the foe attempts to move to a position where the mouser is no longer in its space, the movement provokes an attack of opportunity from the mouser. This deed replaces opportune parry and riposte."

Okay, so what, exactly, doesn't stack? Like if you use Underfoot Assault to get up into homeboy's business you don't get Underfoot's +2 shield bonus and +2 to attacks? Homeboy does NOT take Underfoot's -2 on concentration checks? Why not? Those effects have nothing to do with Mouser's -4 on attacks that aren't the Mouser. There does not seem to be anything that would stack or interfere or even interact with that feat and that deed.

The Cooperative Swarmer feat offer's a +2 shield bonus, and that obviously wouldn't stack with the +2 shield bonus from the Underfoot feat. But even then, the conditions required to gain the benefits of those two feats are completely different.

However, the Vexing Dodger's Limb Climber ability seems to actually get around Underfoot's restrictions... and both could function together, as normal.

All of the other fun feats focused on the swarming trait lack this stupid restriction. So I can't really look to other similar things for context.

[Taja the Barbarian]

Underfoot (BotB) (Combat) wrote:

Source Blood of the Beast pg. 23

You clamber in and around your enemies’ personal space to devastating effect.

Prerequisites: Dodge, swarming racial trait.

Benefit: You are able to share a space with a hostile creature at least one size category larger than you. You gain a +2 shield bonus to your AC while occupying an opponent’s space, as well as a +2 bonus on all attack rolls against the creature whose space you share. Creatures you share a square with take a –2 penalty on all concentration checks. Entering a space occupied by an opponent provokes an attack of opportunity. This ability does not stack with similar abilities providing a creature benefits for being in another creature’s space, such as a mouserACG swashbuckler’s underfoot attack deed.

Special: This feat counts as the Disruptive feat for the purpose of qualifying for other feats that have Disruptive as a prerequisite.

I read the restriction as "If you use this feat, you can't use any other feat or class ability that specifically involves you being in your opponent's space" so yes, you can choose to use this ability or the mouser's deed but never both.

Whether you can change which option you use each round is unclear and is probably a GM's call.

Basically, they don't want you getting multiple sets of bonuses for being in your foes' space: This restriction can be explained as the different abilities actually using different styles that aren't really compatible with each other.

This feat seems to be aimed at Ratfolk who aren't Mousers or Vexing Dodgers but want to enter their foes' space anyway...

[I grok do u]

Does seem unnecessary, but my reading with the restriction is how you enter determines your benefits. Mouser spends panache (no AoO) to share the space and get mouser benefits; Underfoot feat takes AoO to share space and gain benefits. I guess the feat dev didn't want anyone bypassing the AoO with panache nor allowing the mouser to have a free shield bonus or that the mouser get a possible flanking bonus and the feat's +2?

I'd say vexing dodger doesn't have an issue because it doesn't allow you to share an enemy's space except for moving through without an acrobatics roll. You can gain all the climbing benefits while sharing space OR being adjacent to enemy. To get the feat's bonus you'd still eat the AoO before or after the climb check.

Finally, as you pointed out cooperative swarming is for sharing an ally's space, not a hostile, so irrelevant.

On second thought, it might have been to prevent two mouser ratfolk from both entering and sharing a single enemy's space so that they would get both the +2 shield and -4 penalty.
Which raises the question how many ratfolk can swarm that enemy with this feat, and do those concentration penalties stack? X4 per square with rat stack + tunnel rat?