I don’t think there is an actual difference. I feel like this was probably an oversight when they wanted to just use a unified term and they were probably just overlooked.
Treat it as any roll they make is *D.
This enemy is supposed to be enormous! No rule is explicit about this. How I would rule it is: Due to its size, no human or immortal can really wear the Iron Titan armor properly. It would be fantastically used for some vehicles or armor suits. However, if they break it down, they could probably have someone make some Iron Armor.
Not in the Enemies section. HOWEVER, if you look at the Armor List pg. 52, you could treat it as Iron Plate, ignore first hit. Any other Armor is just basic armor, taking 3 hits. You could also just treat all enemy armor the same and only worry about the special properties when the character’s loot it. Essentially, treat all IRON and normal ARMOR as 3 hits when on enemies. If the characters succeed in salvaging some of the armor, they can then treat it as IRON ARMOR PLATE: IGNORE THE FIRST HIT.