Cover incurs penalties to hit something behind something else. This is a shield benefit which many do not know.
Basic Set: Campaigns discusses penalties for varying degrees of coverage such as 50%, on page 407-408. Half coverage is a body part is a mere -2 to hit, but complete cover means it can never be hit. Doesn't really address other intervals. Presumably -1 for 25% and -3 for 75% covered? What about 99%? Advanced rules are needed!