Okay, let's look at the section as a whole for Armor and Clothing -> Armor Rules in
Arsenal.
These rules are all optional—use them if you like, discard if you don’t. They complement the rules for armor given on pp. 148 and 315, SR4.
That means
each rule is optional in and of itself.
Tweaking Armor Encumbrance,
Maximum Armor Modifications,
Armor Suit Capacity, and
Armor Degradation are each a separate rule and you can pick and choose which one you want and don't want in your game.
That being said, the
Max Armor Mod and
Armor Suit Capacity are two separate and unique rules that can't be used together. By using MAM, you're creating a
Ratings capacity are all armor. This means that when you add modifications, you look at the Rating of the modification to "eat up" the Ratings capacity. So, for the example in the book, the Fire Resistance is 5, the Insulation is 3 and the Shock Frills (that don't have a rating) default to 1, giving the 9 Ratings points that fill the armor's capacity. If you use the MAM on Heavy Milspec Armor, that means it can accept a maximum of 24
Ratings points of armor modifications before it "fills up".
The Capacities listed for each armor in
Arsenal are all optional, intended to be used ONLY if you're using the
ASC optional rule. Capacity is meant to make the Full Body Armor suits more like cybernetic exoskeletons (HALO, Metroid, etc.). This was probably placed in the advanced rules because in the basic rules, you had no limit (except your cred) to the amount of modifications you could make to your armor. That idea was fine and dandy when the most powerful armor was Full Body that gave you a 10/8 and was restricted. But I like to think the developers realized the power they were putting into your hands with the Milspec and other suits in
Arsenal and added this rule (and suggesting it's a good "base" rule, even though it is still optional), to keep some balance to the game.
