The Decorated Jacket is a Body armor added in the Summer Camp 4 Update. It can be purchased as part of the Decorated Soldier Set. It can also be obtained by completing the second trial of Summer Camp 4.
Besides a 12% Accuracy increase, you also get a 25% damage boost after reloading a handgun.
A full list of handguns can be seen here.
- You don't actually have to reload the weapon to achieve the damage boost! As long as you press the reload button with a handgun, the ability will activate.