Changes

Unlike all of the other elemental type boss modifiers, Arcane does not add Arcane damage and a purple glow + Arcane damage to the bossesmonster's regular attacks. Instead There are no visual cues at all, until the Arcane is manifested by an monsters cast one of their "Arcane turret" type effect that deals damage to any characters or pets in rangeattack modules. 

There are boss modifiers for all of the damage types, but Arcane clearly presented a naming challenge for the developers. The other elemental damage type boss modifiers all got a one-word name: Poison = plagues[[Plagued]], Lightning = [[Electrified]], Fire = [[Molten]], and Cold = [[Frozen]]. But Arcane doesn't translate so easily. You can envision the developers trying out terms like "Arcane-izedified" or "Arcaned," and finding them wanting. Hence the two word solution, "Arcane Enchanted."  It's odd that the developers didn't go with that format for all of the boss modifiers, just for the sake of consistency. Cold Enchanted, Fire Enchanted, Lighting Enchanted, and Poison Enchanted along with Arcane Enchanted. After all, that's how the [http://diablo2.diablowiki.net/Monster_modifier#Unique_Monster_Bonuses boss modifier naming system] worked in Diablo II.

One wonders if It's possible that the developers considered retroactive changing all difference there is intentional, as the other damage four elemental type boss mod terms bosses manifest it constantly during battle and add it to be consistent: Cold Enchantedtheir attacks, Fire while the Arcane Enchanted, etc...are not glowing purple and only cast their turret attacks now and then?