Should I add my GameObjects as children of ARSessionOrigin?

I understand ARSessionOrigin is used to convert trackables from "device" or "session" space into Unity World Space (?), but to be clear it should ONLY be used for this purpose right? I shouldn't place GameObjects (e.g. a ball with a rigid body) in here?

 

That is generally correct, yes. But there are valid use cases where it is appropriate to put something under the ARSessionOrigin yourself. See my response to a previous question on this.

 

Thanks! Presumably that will apply to objects I want to 'anchor' via ReferencePoints as well? e.g. I'd create a ReferencePoint and attach my object to that?

 

Not necessarily; again it depends on your use case. Making your object a child of the reference point would cause its pose to update automatically, which is usually what you want from a reference point. However, anything under the session origin will not appear to be affected by scale. This is because the camera is scaled by the same amount, so objects under the session origin would not appear to change their size. If you aren't messing scale, then this isn't relevant; just something to be aware of.