One of the central tasks of any device designer is to create the interaction model. The interaction model is the overarching framework that ties the functionality together into a unified whole.
A differentiator is a characteristic that, all other characteristics being equal, would cause a user to choose one device over another.
It helps to consider these device categories, because each brings with it different expectations as to how to engage with the device and what kinds of functionality might be available, not to mention qualities such as durability and cost. Device designers need to be aware of this when considering what form a device should take.