Hmm... well. There's 6 possible types of damage as for weapon attacks as for spells.
"Spell damage" modifier boosts all types of damage (fire/lightning/cold/poison/phys/magic).
"Elemental damage" counting only fire/lightning/cold/poison but not phys/magic.
So "phys/magic" counts physical and magic damage sources like paladin Vessels, Balefire, Magic missiles, Singularity (don't miss with Weapon Damage Modifier skills- they won't benefit from it).
But it won't work on feedback skills with damage based on some of your stats (mana, life, etk.) like Psionic storm, Arcane torrent, Immersion.