Could Devs please explain in detail these rules and variables?

[KLS]

Not sure on the first two xp mods, can look into it if I can remember tomorrow

AcRandom - what does this do? how does it work?
ACreduction - what does this do? how does it work?

These are server vars governing the old AC system, there's also ACfail.. it's an odd odd calculation and it's too late for me to do math right now, it can be found in attack.cpp under mob::meleemitigation() if you're really interested.

Aggro:PetSpellAggroMod
Aggro:SongAggroMod
Aggro:SpellAggroMod

Basically how much aggro these spells make.

Defaults 100 for spell so all spells cause normal aggro.
33 for songs, songs are reduced to 33% of normal aggro
10 for pet, spells cast by pets are reduced to 10% of normal aggro

Spells:ResistChance
Spells:ResistMod
Spells:PartialHitChance

-Chance to resist a spell given no resists and equal level.
-Your Resists * ResistMod = chance to resist without factoring in level, so a 130 resist at the default 0.4 would give you a 52% chance to resist assuming completely clean conditions.
-The % chance a partial capable spell will be partially resisted when it fails the resist check. It's a default of 0.7 or 70%. So a partially capable spell has only a 30% chance to be fully resisted when it fails the standard resist check, nukes roots fears and the like fall under this.

Character:HealOnLevel - does this means char gets 100% heal when he dings? What about 100% mana?

Basically was a feature in the hardcoded features file yes that's what it means, mana on level happens no matter what there was no previous rule governing it but I suppose we can make one someday.

Combat:PetAttackMagicLevel - what about mobs who immune to non bane weapons?

SOL, nothing to help them out except handing them a bane weapon atm.