Customized armor is made specifically for a certain being. The smith who is working it takes a being's measurements and crafts the armor to those specifics.
Customized armor gives the following benefits:
~ +1 to the maximum dexterity bonus. Since the armor is crafted to your specifics, it allows you more freedom to move.
~ -2 to the armor check penalty. Again, it is made to your measurements, and allows greater mobility.
~ - 5% to the arcan spell failure. Same explanation.
All customized armor is masterwork, but the effects of masterwork don't stack with the bonuses from being customized, although other special qualities such as mithral do. Customized armor acts as normal armor to everyone execpt the person who it was made for.
Any armor can be customized, except for full plate, with must be customized. It costs 600 gold pieces plus the cost of the masterwork suit of armor.