My apologies, I don’t normally check this subforum so I missed this one… but (belatedly):
Superior Armour X - An attack must beat this unit’s defense by X or more in order to score two hits against it.
Normally to score two hits with one attack you only have to get 1 more than the units defence. With Superior Armour X you have to beat it by X instead.
For example to score two hits to damage a Chaffee I would need 6 successes (Chaffee is Defence 4, Superior Armour 2).
Let’s say the Chaffee was only disrupted by my attack, so next turn my attacker fires again - I would need 5 successes against the Chaffee now since it’s defence is down to 3 because of disruption, and Superior Armour 2.
Once again I’m unlucky and only disrupt it, or more accurately - only cause one hit; however I have a second unit able to fire on the Chaffee. I roll 5 successes against it; this beats the Superior Armour 2 so I score two hits. It already has one hit, so with two more it will be destroyed during the casualty phase.