If Change Pitch Rotation is enabled, the object’s Z axis always points at the camera.
If the option is disabled, the pitch angle is not changed. Suppose you are using a 2D tree in the scene (a plane with an alpha map). This tree must constantly face the camera, but also, its Pitch value must not change (i.e. it must remain perpendicular to the ground).
In such cases, disable Change Pitch Rotation and enable Camera Dependent (Basic Properties tab).