While the text might seem to be a bit more complex than necessarily (this is something that Shadowrun is quite (in)famous for) it is technically not at all wrong or subject for an errata.
This is how it works:
Each piece of armor have a Defense Boost. Sometimes they stack (like a jacket and a helmet). Sometimes they don't (like armored clothing under a coat).
Then you calculate the Armor Rating of the target. The Armor Rating is the sum of all individual Defense Boosts that do stack plus the Dermal Deposits quality, the innate Mystic Armor power, the innate critter Armor power (but not the Armor spell or Combat sense spell), the Orthoskin augmentation (but not Bone Lacing or Dermal Plating). This distinction is important because of armor related mechanics like Hardened armor, Touch based attacks, Cold based attacks, Chemical based attacks and the Petrified status.
Next step is to calculate your Defense Rating. The Defense Rating is generally calculated as your Armor Rating from above plus your Body Rating plus Armor Spell, Combat Sense spell (but not Combat Sense power), Mystic Armor adept power, Bone Lacing augmentation, Dermal Plating augmentation, Cover status effect and Stilled status effect.
For most intents and purposes you only ever just worry about the Defense Rating and in most cases it will be a very static value. Just write it down once and then don't worry about anything I wrote above. But since the question about errata came up I just wanted to set the facts straight.
