Need help with Spell

[Dragon 661]

In the spells Bone Armor (I have added it below), what does the "(AC 17)" Mean???

Bone Armor
Necromancy
Level: Clr 2, Nec 1
Components: V, S, M
Casting Time: 1 action
Range: Caster
Effect: Protect caster
Duration: 10 minutes/level
Saving Throw: None
Spell Resistance: No
This spell creates a layer of bony ribs on the caster’s arms,
legs, and torso, as well as a skull-like helmet. The effect is to
provide the caster with a +7 natural armor bonus (AC 17)
against all physical forms of attack. The bone armor is
destroyed by the attacks it absorbs; when it has absorbed hp of
damage equal to twice the caster’s level, the armor falls into
useless brittle shards.
Material Component: A rib bone from a humanoid slain
within the last seven days.

[Megan Robertson]

If the caster is standing there buck-nekkid, he's AC 10 (plus a Dex bonus if he has one). He then casts the spell and becomes AC 17 (plus any Dex bonus) as the spell adds a +7 to his AC.

[Ross Byers]

This is a duplicate thread. Please discuss over here. Thread locked.