AR Plane Manager allows the app to detect horizontal and vertical planes in the device's camera's view. AR Raycast Manager allows the app to identify which horizontal or vertical plane a raycast from the direction of the camera would strike. Our script fires a ray every frame via the Update() method.
By tagging AR Camera as Main Camera, we are able to easily get a reference to the camera using Camera.main.
By placing the instance object of our script class as a component of the AR Session Origin GameObject, we are able to get a reference to the ARRayCastManager sibling script component by using GetComponent<class>() in the Awake() method.
The only public variable we have to set for the instance object of our PlacementIndicator script class in the AR Session Origin GameObject is the GameObject of the plane that's displaying our circle image.