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See the official rules here:
www.d20pfsrd.com/gamemastering/other-rules/piecemeal-armor

We're using the rules for piecemeal armor in this game; and although you'll most likely choose to buy armor in full sets, this presents us with other options:
  1. You can buy part of a heavier type of armor, for example breastplate, without having to buy the full set.
  2. You can replace part of a set of armor if it gets damaged without having to buy the full set.
  3. Armor you find in treasure hoards may not be in full sets.
  4. When a combatant sunders someone's armor, it damages a piece of the armor, instead of damaging the whole suit.
  5. When a combatant suffers a damaging attack that inflicts an injury or critical injury to an armored hit location, depending on the type of damage, the armor may receive the damage instead of the character. In this case, the character suffers subdual damage equal to that inflicted by the attack and may be either staggered or rendered unconscious (and therefore helpless against further attack).

Be sure to check out the Masterwork, Special Material, and Magic Armor section in the rules entry linked above for details on high-quality armor.