This is one of the most annoying and awkward pieces of 3.x rules
You don’t get enhancement bonuses to AC, you get enhancement bonuses to armor bonuses
So, armors grant an armor bonus to AC. Seems simple enough.
Then we have enhancement bonuses. You could, in theory, have an enhancement bonus to AC; if you did, it would stack with an armor bonus, since they are different bonuses. However, as far as I know, there is no effect in the system that gives a creature an enhancement bonus to AC.
No, instead enhancement bonuses are applied to items. When you wear magically-enhanced armor, you don’t have an enhancement bonus, the armor has an enhancement bonus – to the armor bonus that it gives you. So in short, your AC doesn’t “see” an enhancement bonus at all; it just sees a (larger) armor bonus.
A similar example exists in the amulet of natural armor’s “enhancement bonus to your natural armor bonus.” This time, the enhancement is on the person wearing the amulet, but it’s not an enhancement bonus to his AC, but an enhancement bonus to his natural armor bonus. As far as the wearer’s AC is concerned, it sees no enhancement bonus, just a larger natural armor bonus.
And so it is with magic vestment: it applies an enhancement bonus to an item’s armor bonus (including an armor bonus of +0 from a non-armor). Your AC still only sees a larger armor bonus, not any enhancement bonus.
Thus you might have a shirt that has a base armor bonus of +0, to which it receives a +4 enhancement bonus. The wearer of that shirt gets a +4 armor bonus. If they are already wearing full-plate, the +8 armor bonus of that supersedes the +4 armor bonus of the shirt, so the shirt becomes pointless. On the other hand, magic vestment could be cast on the full-plate, giving it a +4 enhancement bonus, improving its armor bonus to +12. If it had instead been a +1 full-plate, the +4 enhancement bonus would supersede, not stack with, the +1 enhancement bonus, so the armor bonus to the wearer’s AC would still be +12.
Fortification is a magical enhancement.
This suit of armor or shield produces a magical force that protects vital areas of the wearer more effectively. When a critical hit or sneak attack is scored on the wearer, there is a chance that the critical hit or sneak attack is negated and damage is instead rolled normally.
Fortification Type Chance for Normal Damage
Light 25%
Moderate 75%
Heavy 100%
The base armor could be any type. It needs to be magic armor, ie, have at least a +1 bonus.
In addition to an enhancement bonus, armor may have special abilities. Special abilities usually count as additional bonuses for determining the market value of an item, but do not improve AC. A suit of armor cannot have an effective bonus (enhancement plus special ability bonus equivalents) higher than +10. A suit of armor with a special ability must have at least a +1 enhancement bonus.
You'll need to ask your DM for more details on your specific armour.
Best Answer
If you are referring to the table, Armor Special Abilities, please note, the "+5 bonus" to the right of "Spell resistance(19)" is under the column heading, "Base Price Modifier". This is merely informing you that spell resistance(19) has the equivalent cost or value of a +5 enhancement. Thus if the armor was a +4 chain shirt with spell resistance(19), the total value of the magical attributes would cost the equivalent of a +9 enhancement bonus to armor, or 81,000 gold pieces, but it would only grant a +4 enhancement bonus to the armor's armor bonus.
To be complete, the armor bonus is the total of the armor's armor bonus plus the enhancement bonus to the armor bonus (sounds odd, but this way they stack). Thus for a chain shirt, the item's armor bonus is 4, and you add any enhancement bonus to the armor bonus, so a +4 enhancement would make it a total of +8 to the wearer's armor class. Shields work the same way; the enhancement bonus is added to the shield bonus. Thus you can only have one armor bonus and one enhancement bonus to your armor bonus. The same is true of shields, you may only have one shield bonus and one enhancement bonus to your shield bonus.
Also, as mxyzplk points out, from magic armor
So even though the abilities have a cost equivalent to an enhancement bonus, the item must have an actual enhancement bonus to also have a special ability.