Billboard always faces the camera plane, regardless of rotation.
"BILLBOARD_ORIENTATION_FACE_CAMERA_GROUND"
Billboard lies parallel to the ground and rotates around it's anchor point to face the camera. SetRotation can be used to offset the final angle of the billboard.
"BILLBOARD_ORIENTATION_GROUND"
Billboard lies on the ground, orientation does not depend on the camera position.
Billboard always faces the camera plane, regardless of rotation.