Question about Damage modifier

[Kogan]

Quick question:
Have just found
the cursed +3 leather armour of Inviolability (worn) {+Blink MR Dam-4 SInv}
Now, does the Dam-4 only apply to the meele damage (I'm an caster, so that wouldn't matter),
or does it also affect the spell damage ?

[Sometimes]

I think Dam and Acc modifiers in randarts work like Rings of Slaying, so only melee or missile weapon damage would be affected.