Basically Defense comes in two varieties.
1: Typed (Smashing, Lethal, Energy, Negative, Fire, Cold, Psi)
2: Vectored (Melee, Ranged, AoE)
Most incoming attacks have flags for both Typed AND Vectored damage.
Very VERY few are "Pure X".
And the way Defense works, the game uses the highest applicable value.
So if you have Smashing/Ranged damage.
Your S/L Defense is 0% and your Ranged Defense is 25%, it'll use your Ranged Defense.
If you get Energy/PBAoE damage.
Your Energy Defense is 30% and your PBAoE is 22%, it'll use your Energy Defense
And yes, if it throws a damage combo that you have NO Typed OR Vectored Defense to? You're hating life and hoping you have some Resistance.