I believe it acts like Enhanced Defense from skills. So if you have <insert defense-boosting skill here> that grants you 100% defense and a jewel in your weapon that grants 20% defense, your base defense will be boosted by 120%.
I could be wrong, though. I'd wait for somebody to confirm/disprove this first.
There are no enhanced defense jewels, only straight +defense. Items with straight defense (including charms) are just classed as another item source for defense to be added to armor defense + glove defense + helm defense + etc. This is then increased by skill defense %.
Still it is possible to get off-armor ED% (eg The Mahim-Oak Curio) and it would indeed calculated as Nimbostratus indicated above.
In this case, enhanced defense = %def, i.e. a bonus measured as a percentage of the base defense of an item, and +def is just what it says, a number as a bonus to the total defense rating, to be added after %ed is applied.