Short answer: yes.
More detail:
All defense "soft caps" at 45%. Defense falls into two general categories: positional or typed.
The aspect where it becomes more complex for typed defense is: an attack is melee *OR* ranged. It can't be both. But an attack can be smashing and fire types at the same time.
So, say you have 45% smashing/lethal defense and 30% fire defense. You get hit with one of various fire attacks that is listed as smashing and fire. The game automatically picks the better of your defenses. You would get 45% defense vs a smashing/fire attack. So, answering your precise question: you would get that 45% defense if any part of the enemy attack is smashing or lethal. Many attacks are all smashing or all lethal, but your defense would be good against 'split' ones like the hypothetical smashing/fire attack I mentioned.
And, it can get even worse. Some attacks have no positional element. Some attacks are 'special' and ignore all defense types. 
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All attacks have flags that indicate what defense they check. You can see these under "attack types" in real numbers: e.g. Ice Blast is flagged ranged, cold, smashing.
When the attack is used against you the game checks to see which of your applicable defenses is the highest, and uses that defense. e.g. you have 0% fire/cold defense, 20% ranged and 45% smashing/lethal. Against the Ice Blast attack, 45% defense would apply.
Some attacks only have a positional or typed flag, but not both (e.g. Toxic attacks only have a positional flag and Mind Control attacks only have a Psi damage flag).
No, for typed defense (smashing, lethal, etc.) the game checks against the attack type to determine your defense against it, and it uses the highest value applicable. For example, if you were attacked by an energy blast (smashing/energy damage) and had 30% smashing defense and 12% energy defense, the game engine would calculate your chance to be hit based upon your smashing defense, the highest in that case. Later, if you were on the receiving end of a pure energy typed attack, you would have only 12% defense.
The soft cap for either typed or positional defense is 45%. Even then, there are some attacks that are typeless (Hamidon) and some that lack positional tags (most Mind Control powers). Also, some enemy attacks may not have what seems like the correct type.
EDIT: Hurr... what them other two said.
Extremely useful info from the 3 of u, +3
I have always been confused about this. I know how melee and ranged def work but i never understood the separate parts like energy , smashing, lethal etc. This is my understanding.
Smashing /Letha Def softcaps around 45% This means critters will have about a 5% chance to hit me if their only attacks are smashing or lethal, is that true?