Arcane Subtlety does NOT affect your Spell Hit chance.
Arcane Subtlety gives you 10 Spell Penetration.
Spell Penetration subtracts from the enemy's resistances, those 5 schools of resistance which you can see on the right of your character model in your character window when you press the "C" button ingame.
Elemental Precision adds to Spell Hit, not Spell Penetration.
Here's the explanation between the two:
Whenever a spell is cast, there are TWO checks by the game on whether the spell will hit or not. The FIRST one is based upon level difference and Spell Hit chance. Put simply, with NPCs, your character has a base 96% chance to hit a same-level NPC with a spell. If the NPC is a level higher, it is a 95% chance. 2 levels higher? 94% chance. 3 levels higher? 83% chance. (And something like 72% for 4 levels higher than you).
Spell Hit increases your chance to hit on this scale. So, you need enough Spell Hit Rating to bring your Spell Hit chance to its maximum, which is 99%. At level 70, to bring your Spell Hit chance to 99% on boss targets, who are always treated as 3 levels higher than max player level, you need at most 202 Spell Hit Rating, brought down to 164 Spell Hit Rating after taking three points in Elemental Precision. For PvP, you won't need more than 3% Spell Hit chance on same-level targets.
The SECOND check by the game is based on the target's Resistance. Unlike the level-based resist check, the Resistance-based resist check can yield results of partial damage. Direct damage spells that are NOT binary spells (binary spells are spells that have non-damage effects from them) have a chance to do 100% damage, 75% damage, 50%, 25%, and 0% damage. At level 70, if you have 210 resistance in a school of magic, spells cast at you from that school will have around a 50% chance of doing 50% damage to you, like a 20% chance to do 25% damage, a 20% chance to do 75% damage, stuff like that.
Spell Penetration's value is simply how much of your enemy's resistance values you ignore. If your enemy had 40 Arcane Resist, 9 Fire Resist, and 0 Frost Resist, and you had 2 points in Arcane Subtlety, your spells would hit and have its second resistance check as if the enemy had 30 Arcane Resist, 0 Fire Resist, and 0 Frost Resist.
I used colors to try to show distinctions with consistencies.
Edited, Apr 29th 2008 9:08pm by Raglu
Replaced the painful purple with light coral.
Edited, Apr 29th 2008 9:08pm by Raglu