Armor as Damage says:
Armor in this system keeps all of its normal statistics and qualities, but its armor bonus (including any enhancement bonus added to armor bonus and natural armor bonus ) is converted to DR/armor. The DR an armor provides is equal to its total armor bonus...
... plus some about how it scales with hit dice.
Further,
DR/Armor: This type of DR blocks the damage of all attacks that would normally be affected by DR, based on the composition of the armor (see Table: Armor Composition and DR). Unlike most forms of damage reduction, DR/armor stacks with other types of DR. For instances, when fighting a skeleton with DR 5/bludgeoning and DR 4/armor (+2 for armor, +2 for natural armor ), the skeleton’s DR/armor reduces 9 points of damage from non-bludgeoning attacks, and 4 damage from bludgeoning weapon attacks. Magic weapons and attacks from Large or larger creatures bypass the DR 4/armor, but not the DR 5/bludgeoning.
(emphasis mine)
From that table, nonmagical armor grants DR/magic. So, nonmagical armor that would normally grant a +5 Armor bonus to AC instead grants DR 5/armor(magic).
A creature with DR 10/magic and a +2 natural armor bonus instead gains DR 2/armor(adamantine).
That creature ("Bob") wearing that armor would now have:
- DR 5/armor(magic)
- DR 2/armor(adamaintine)
- DR 10/magic
... and gets to apply them in order.
Bob gets attacked several times, by differing weapons (all wielded by a normal human with a terrific str bonus). In all cases, the attacker hits and rolls 20 damage.
- a nonmagical, no-special-materials sword: all DR sources apply, Bob takes 3 damage
- a magical, no-special-materials sword: only DR 2/armor(adamantine) applies; Bob takes 18 damage
- a nonmagical adamantine sword: both DR 5/armor(magic) and DR 10/magic apply sequentially, so Bob takes 5 damage
- a magical adamantine sword: all DR sources are bypassed, Bob takes 20 damage
Bob then takes a few levels of Barbarian and gets DR 1/- from that class.
Bob now has:
- DR 5/armor(magic)
- DR 2/armor(adamaintine)
- DR 10/magic
- DR 1/-
Now, the normal DR overlapping rules kick in for the DR 10/magic and DR 1/- (ie., magic swords bypass the former so the latter applies, nonmagical swords don't bypass the former (which is better for Bob), so the latter doesn't apply). But, the DR/armor still stacks. Against that same human's attack:
- a nonmagical, no-special-materials sword: all DR sources apply, DR 10/magic trumps DR 1/-, Bob takes 3 damage
- a magical, no-special-materials sword: only DR 2/armor(adamantine) and DR 1/- apply; Bob takes 17 damage
- a nonmagical adamantine sword: both DR 5/armor(magic) and DR 10/magic apply sequentially, with DR 10/magic again trumping DR 1/-, so Bob takes 5 damage
- a magical adamantine sword: all DR sources are bypassed except DR 1/-, Bob takes 19 damage
In short: gaining DR in addition to DR/armor is still useful, subject to the same caveats about DR overlapping that have always been in place. DR/armor is "simply" a second level of DR that applies before those other sources. Actually, gaining DR other ways is probably more important when using DR/armor, since you're likely to be getting hit a lot more than without that optional rule.