Armour Rating (Seen as AR on write-ups)
When firing on a target wearing body armour, your attack roll must be higher than the AR of the Body Armour in order to harm the person inside.
If your beat the AR of an SDC Body Armour, the wearer takes half the damage dealt while the armour takes the other half.
If you beat the AR on an MDC Body Armour the wearer takes the equivlent in SD that the ewapon deals in MD while the the armour takes half of that in MD (IE a 3d6md will do 3d6 sd to the wearer and half that amount in MD. so 10 sd to the wearer would do 5md to the armour. Yes bit convoluted but it shows the armours is doing its job in protecting the wearer but still takes a beating for it. It is not as convoluted in practice it seems in reading it)
If you do no beat the AR, then the armour takes the full damage accordingly.
For me beating the AR is punching through it not "finding where the armour isn't" and thus the armour even if you beat the AR will still absorb some of the damage.
This is different from Body Armour and is treated exactly as it is in the rules as printed.
If you do not beat the AR you do no damage.
If you beat the AR you do full damage.
Armour Description (Seen as Armour on write-ups)
This is where the item will have an explanation of what it can stop regardless of being hit or if the AR is penetrated. Basically whatever type of damage rating/caliber is listed here, no damage can be done by it.
Example - stops up to and including the equivalent of a standard 20mm round (2d4x10+10sd/2d4+1md).
Which means anything upto and including the equivalent of a standard 20mm round cannot damage the target regardless of beating the AR or otherwise.
MDC vs SDC
When firing MD weapons at SDC armours and vehicles multiply the damage of the MD weapon by 10.
When firing SD weapons at MDC armours and vehicles divide the damage of the SD weapon by 10.
AR still applies as per normal.