The content of this document is divided into two parts. — This document analyses the overall configuration of an industrial digital twin system, and proposes a three-elements architecture, focusing on the twinning interface between the physical twin (PTw) and industrial digital twin (iDTw). — The characteristics, and the visualization elements and visualization fidelity of iDTw are analysed. This document: a) analyses the twinning interface between the PTw and iDTw; b) proposes a three-elements architecture; c) analyses the visualization element and its fidelity, which is a key component of the interface among the three-elements architecture; d) analyses the elements that constitute an iDTw system to understand the unique properties of iDTw; e) explores the differentiation from cyber physical systems (CPS) or augmented reality (AR), which are similar to existing concepts of iDTw. This document excludes: — applications of iDTw; — implementation of iDTw.