Some ball joints must be installed in a specific orientation to function correctly. These ball joints are designed to allow for a large amount of ball stud swing in the cross-car or side-to-side direction as the suspension travels up and down.
The trade-off is that they have less swing in the opposite or front-to-back direction. As a result, directional ball joints have an index mark on the housing in the form of a hole, dimple, notch, arrow, etc.
These marks indicate which way the ball joint has the largest amount of swing and must point outboard, toward the wheel of the vehicle when installed. Failure to install a directional ball joint in the correct orientation could result in damage to the ball joint due to over travel of the ball stud.