# A Systematic Review of the Trends and Advances in IFC Schema Extensions for BIM Interoperability
## 3 Evolution, Architecture, and Implementation of the IFC Schema  
### Evolution of the IFC Schema  
The IFC is an open and neutral file format for the exchange of project lifecycle information within the AEC/FM industry. It is an international standard (ISO 16739-1:2018 [11]) designed to represent digital descriptions of the built asset industry, with the goal of promoting vendor-neutral, or agnostic, and usable capabilities across a wide range of hardware devices, software platforms, and interfaces for many different use cases [24]. To meet the ever-growing needs of the industry, the IFC schema has evolved to encompass new domains as well as related concepts, processes, and related technologies. Figure 2 shows the versions of the IFC schemas released to date. IFC 1.0 was initially released in 1997, while its most current version IFC 4.3 has been under ISO's Draft International Standards (DIS) voting since 2021. Throughout this period, the schema has been revised multiple times. These revisions are marked by continuous updates that involve the major introduction of new entities and features, refinement of existing functionalities, as well as minor bug fixes. Figure 1: Research methodology. Figure 2: Evolution of the IFC schema. The varying degrees of updates are represented through the'major-minor-addendum (Add1, 2, etc. )-technical corrigendum (TC1, 2, etc.) ((1) 'Major release' encompasses expansions or deletions in scope; (2) 'Minor release' includes feature extensions with guaranteed compatibility for core schema, but not for other definitions; (3) Addendum comprises improvements to existing features with guaranteed upward compatibility; (4) Technical corrigendum entails enhancements to documentation.)' notation in which its constituents serve as ancillary qualifiers for the specific version. The revisions not only included additions but also rectifications, consolidations, and deprecations. Decisions surrounding the official release of an update are reached via a consultation vote conducted by the bSI Standards Committee, the body responsible for schema revisions. An update that gains approval is termed 'official' and is rolled out for implementation. Conversely, if an update is deemed to require further enhancements and is thus omitted from the official version, it is classified as 'withdrawn'. IFC 1.0 was primarily focused on defining the architecture domain, together with the building services and the construction management domain. With IFC 2, new schemas were added for the structural domain (_IfcStructuralAnalysis_ and _IfcStructuralElement_) and additional building services (electrical, heating, ventilating, and air conditioning (HVAC), etc. ), cost estimation, and construction planning. By IFC 2X3 TC1 (2007), the IFCs started identifying ways to incorporate data with geographic information systems. A major update occurred with IFC 2X3 to IFC 4 to support new geometric shapes, parametric functions, coordinate system information for GIS linkage, linkage with infrastructure models, etc. These additions and refinements were vital in eliminating structural ambiguities, reducing the complexity of the file format, and boosting the compatibility of the IFC schema [25]. The development continued with IFC 4.1, which enriched the schema with alignment expressions specifically designed for infrastructure structures. The subsequent version, IFC 4.2, expanded the scope of the schema to incorporate elements related to bridge structures. IFC 4.2 also defined a separate spatial hierarchy for infrastructure, specifically for bridges, by defining entities such as _IfcFacility_ and _IfcFacilityPart_. However, these two minor updated versions were withdrawn to make room for a more comprehensive redevelopment, with an eye on the broader infrastructure domain. The most recent version, IFC 4.3, currently in its final stages of development and on the cusp of an official release, was a significant step forward.