You have mastered the use of one type of shield.
Benefit: Choose one type of shield (buckler, light, heavy, or tower shield). With the selected shield, you gain a +2 bonus to your Armor Class against critical hit confirmation rolls. In addition, you may add your base shield bonus (including the bonus from Shield Focus but not including enhancement bonuses) to your CMD.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new type of shield.
Advanced Player’s Guide. Copyright 2010, Paizo Publishing, LLC; Author: Jason Bulmahn.