Masterwork

A Masterwork item (or Foundation item) is an special equip item (armors, weapons and jewels) which is created with a certain rate when crafting.

On L2Retro, the rate is 100% which means that ALL crafted equipment are foundation.

This special items gives bonus stats when equipped depending on the items itself. For example, if you have a vesper set with gloves foundation, you get the vesper bonus + the foundation gloves bonus.

Read this!!!
Please look at the following table to know when a foundation bonus will be applied or not.

Additional Observations
The armor piece will get a bonus similar to the armor bonus of the normal set, plus a bit extra. So for pieces of a robe set, the extra bonus is PDef, for parts of a LA set, the bonus is extra evasion, and for pieces of a heavy set the bonus is extra HP.  If you have a complete set of masterwork items, you get a bit higher bonus than the standard set, but no attribute modifications. For example, a complete masterwork Dark Crystal Robe Set gives a total 18% casting speed increase instead of the normal 15%, but it does not modify the attributes WIT +2, MEN -2, so that the final result is that you get less casting speed with the masterwork set than the normal set. I have already read many posts complaining about this "feature".