The save DC is [x-ability] based-?

[Hiro Quester]

What does this phrase mean?

E.g. When I summon a bat swarm, the swarm has this as an Ex attack:

Distraction (Ex)
Any living creature that begins its turn with a swarm in its space must succeed on a DC 11 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.

Fort saves are constitution based. So what does this mean? Isn't it redundant, since the DC fort save is already listed?

The only reason for it I can think of, is that if the swarm's CON is raised, the DC would increase. Is that it? If so, by how much? A +4 from Bear's Endurance would raise it by 2?

[Crazysaneman]

It means that you calculate the save DC off of the creatures constiution, so if the bat's Con goes up so does the save DC.

Also, it has nothing to do with the type of save to be made, you can find some Will or Reflex save attacks that use the creatures Con or Dex (for example) to calculate the final DC.

[Flickerdart]

The only reason for it I can think of, is that if the swarm's CON is raised, the DC would increase. Is that it? If so, by how much? A +4 from Bear's Endurance would raise it by 2?
That's exactly it. Conversely, if the creature is poisoned, its Constitution (and thus the DC) might decrease. It also helps when adding templates to the base creature, since its stats will go up and down.

[Malcador]

The formula for monster ability DCs is 10 + 1/2 HD + [ability] modifier (plus occasional other modifiers from Ability Focus and such). The phrase "The save DC is [ability]-based" tells you which ability modifier is used to calculate the DC.

[Hiro Quester]

Ah. That makes sense. Thanks, everyone.