Thank you for getting back to me. In the Unity 3D "Scene" pane, I have setup my Vuforia target game object. I can see the image assigned. All is well.
Then I add a 3D model to the Hierarchy. I make that 3D model a child of the target image game object. In the Unity 3D Scene pane, I position that 3D model on the Vuforia target game object. Great! Works fine. This is tested in the Unity editor using my laptop's built in camera (holding up the target paper), as well as in an APK on a phone.
Then in the Unity 3D Scene pane, I move the 3D model off the Vuforia target game object in the X or Z direction. (not Y, not going up). That is, for lack of a better metaphor, the 3D object is north of the top edge of the Vuforia target game object, at the same Y position of 0. Point being, I am not moving the 3D model straight up "off" the target, but to the side of it, at the same plane or Y value.
This doesn't work as I would expect. I would expect that when I put my target print out on the floor, and point my phone to the target, the 3D object would display at scale and position in relation to the target image (as I see it in the Unity editor). It doesn't. The object does display, but it's just floating and not anchored to the target print out.
Is this a supported use case? At the moment my 3D model isn't very tall, in relation to the size of the target image (a few inches), but ideally the 3D model would be 5' tall in relation to the real world. Right now I am just experimenting having any object off the target to see what happens.