Ok, so in the Age of Rebellion core book pg 230 under the grey section talking about Strain Damage and Critical Injuries, it talks about applying a Critical Injury from stun damage. However, nowhere in the book that my group could find talks about applying a critical injury when the strain taken is equal to the strain threshold. So, is that a GM choice or what? In another book somewhere? Any guidance would be wonderful.

Thanks!

There is no auto crit for reaching Strain threshold. The gray box is talking about when a critical injury can be triggered by Advantages or Triumphs via the roll, but the shot is a stun shot only inflicting Strain and ideas of how that might occur.