The posterior group, is also referred to as the rear head of the shoulder muscles. Unlike the lateral and anterior heads, the posterior delt originates from the scalpula and is attached to the humerus. The main function is to move the arm away from the body, specifically to the rear. In addition, it is also your primary shoulder hyperextensor.
The rear deltoids are often a secondary muscle in many back exercises that involve the Teres Major and Rhomboid muscles. You can effectively target this group with direct exercises as well.