I can't find any difference in the rules concerning light, medium, or heavy armor. Is there any difference?

Attack Phase: Your defense roll. page 23 of rules.

The weaker your armor the higher roll you need to defend.

Hammerhead 4+
particalbow 4+
control corvette 5+
carrier 6+
fighters 6+

Brad

so it's a complete redundancy to the DEF on the stat card? Weird.