X-CAD: a stand-off semantic annotation architecture for model-based definition

With the widespread digitalization of manufacturing processes, Model-Based Definition (MBD) has emerged as a key enabler for embedding machine-readable semantic information in three-dimensional (3D) product models, thereby supporting a single authoritative source of truth across the Product Lifecycle Management (PLM) pipeline. However, the broader adoption of MBD remains constrained by the lack of accessible, customizable, and practically deployable data architectures capable of representing semantic annotations beyond standardized Product and Manufacturing Information (PMI) and Geometric Dimensioning and Tolerancing (GD&T). To address this gap, this paper builds on the eXtended Formats (X-Formats) family and introduces eXtended Computer-Aided Design (X-CAD), a stand-off semantic annotation architecture for semantically enriched 3D Computer-Aided Design (CAD) models. X-CAD combines established geometric carriers, including STereoLithography (STL) for tessellated geometry and the Standard for the Exchange of Product Model Data (STEP) for exact Boundary representation (B-rep) geometry, with structured semantic annotations encoded in JavaScript Object Notation (JSON). The proposed architecture supports multi-level semantic enrichment across geometric entity levels, including points, edges, surfaces, volumes, and file-level annotations. It also organizes semantic content into reusable layers associated with specific domains, such as design, manufacturing, assembly, inspection, and maintenance. This paper formalizes the X-CAD semantic backbone, introduces carrier-specific instantiations for STL- and STEP-based workflows, and presents proof-of-concept tools to illustrate the practical implementation of the proposed approach.