Construction is looking closely as Industry 4.0 paradigm (I4.0) transforms many processes in Manufacturing. Technologies associated with I4.0 were born out of the ever-present necessity of automation (and integration) on the shop floor, as well as of better management in product lifecycle with computer-aided software. Although indeed relevant outside Manufacturing, it is yet not clear how to transpose and apply some I4.0 technologies - for instance, Digital Twins (DTs) - outside the context of a factory or of complex, one-of-a-kind products. Many researchers and software companies from Architecture, Engineering, and Construction (AEC) and Facilities Management (FM) sectors are already working with an ill-defined concept of a DT, and some difficulties had arisen in dissociating it from Building Information Modeling (BIM). Without a direct counterpart outside AEC/FM, the practice of BIM could be likened to Product Lifecycle Management (PLM). Aiming at creating a clearer picture of where, when, and what should be a DT on AEC/FM, through literature review, clustering of common terms found, and a conceptual engineering approach, the present work develops a concept of DT, and layout differences between it and BIM model. It is advocated that DTs should be more about "functional models" for simulations than "product models" for information visualization and organization. As a result, DTs could be used to predict "behavior", and thus enhance and transform management, operations, and maintenance practices. In doing so, there is yet a set of challenges that need to be addressed before one could create and employ properly Digital Twins in Construction.