How is a Spectacles spatial app different from a notification display?
A spatial app can involve object placement, scene relationships and interactive rendering. A notification display may only need to deliver a short message reliably. Those are different engineering tasks even though both show digital information.
Match complexity to the user's action
We would not require room placement just to say that a reply is ready. Conversely, an instruction attached to a physical object needs a spatial relationship that a head-following text card cannot supply on its own. The task determines which capabilities earn their complexity.
Compare a maintenance label beside a machine with a calendar reminder. The first benefits from knowing which object it refers to in the scene. The second may be more useful as a brief accessible cue. Choosing between them is a product decision before it becomes a platform decision.