Late again, guess I've got to keep working on my typing speed. Anyways. . .like Saga Edition's "saves", d20 rethought's four base bonuses (Combat, Will, Fortitude and Reflex) each equal half your character level plus a class bonus that ranges from 0 to +2 (multiclassing means taking the higher class bonus in each case). Each of these bonuses is added to a corresponding defense, which starts at 10. More on the applications of each bonus after the link.
-Your base combat defense uses your Grace score and protects you from attack checks.
-Your base will defense uses your Intellect score and protects you from mental influence and trauma.
-Your base fortitude defense uses your Stamina score and protects you from physical afflictions of all types.
-Your base reflex defense uses your Wits score and protects you from hazards where your only option is to get the hell out of the way.
-Your base reflex bonus also is used for initiative checks, again with Wits.
-Your class combat bonus is added to all Grapple, Guard, Overpower, Trip, and Weapon Group checks.
-Your class will bonus is added to all Presence checks.
-Your class fortitude bonus is added to all Endurance checks.
-Your class reflex bonus is added to all Acrobatics checks.
Neither Brawn nor Spirit are used for any defense- on the other hand, these scores do determine your Wound and Trauma points, respectively. As such, low scores in these areas mean a critical hit is more likely to put you out of the action. I'm deliberately trying to avoid statistics that are easily min-maxed; whatever you put that 8 in, it's going to give you a weakness.