What is arcane defense?

[Umpa]

I've never seen this term before, what exactly is it?

Bigby's Crushing Hand:
Once per round the hand makes an attack on the target (Spell DC + 10 STR modifier + d20 vs target's AC). The damage is 2d8 + 1 per caster level. A successful fortitude saving throw halves the damage. Additionally, if a discipline check (target's discipline ranks + arcane defense + skill feats + d20 vs. caster's concentration ranks + focus feats + d20), is failed the target is stunned for that round (bypassing any stun immunities). Duration is 1 round per 2 caster levels.

Its used in this formula, is it spellcraft?

[Binkyuk]

it's a feat very much like spell focus.

[Umpa]

Ok, I found it thanks.

http://nwn.bioware.com/shadows/feats.html#ArcaneDefense

[Umpa]

So I guess my next question is why would this +2 feat against a particular school of magic be in the forumula but SR isn't considered?