Hello everyone,

I am currently working on the state of the art of the digital twin concept, and after seeing the categorization made by Kritzinger, W. et al. (2018) [Article Digital Twin in manufacturing: A categorical literature revi...

• According to the definition, we can speak of a "true" digital twin as long as the data flows between an existing physical object and a digital object are fully integrated and automated in both directions. However, I have found articles where the developed digital twins, instead of acting or controlling directly their respective physical twin, they send warnings or recommendations to a person so that s/he performs the corresponding actions. Hence, to my understanding, there is no an automated flow of information from the digital twin back to the physical object. In this cases, should we consider the developed digital twins as real or as digital shadows instead?
• Regarding the composition of digital twins, is it possible a digital twin to be internally composed of several digital twins? To exemplify this, we could think of a factory that builds a certain product, which is composed of several components that already contain their own digital twin implemented from the supplier. Therefore, we could consider that the digital twin of the final product is, among other things, the composition of all the digital twins of its components. I ask this because I have not seen this property widely developed throughout the literature. Instead, it is common to see that the digital twin is composed of physical and/or data-driven computational models and data that describe in real time the behavior of its physical counterpart.
• Does it have sense to speak about the lifecycle stage in which a digital twin is framed (Design - Development/Manufacturing - Operation/Service - Dismissal/Retire) if it does not represent an object/product? As an example, we could think about the digital twin of an industrial process, of a human being, of disaster management in an Smart City, etc. In my opinion, it does not make sense...

Thank you so much in advance