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.