# A Systematic Review of the Trends and Advances in IFC Schema Extensions for BIM Interoperability
## 7 Conclusions  
This study performed a comprehensive review of preceding IFC schema extension cases within the AEC/FM literature, with the objective of pinpointing research gaps and informing future enhancements in schema extension strategies. Prior to this review, the architecture and implementation methods of the IFC schema were examined, which revealed its inherent limitations. A total of 64 publications conducted over the recent two decades were collected and classified with the goals of identifying the trends with respect to the domains and sectors addressed, their purpose in terms of products and processes, the methods used for extensions, and their implementation and validation approaches. In both the architecture and infrastructure domains, most cases involved creating extensions to represent new processes, although product representation was initially predominant for the infrastructure domain. Defining properties served as the main extension method, with entity extension frequently being utilized concurrently in product-oriented cases. Trend-wise, the architecture domain was initially centered on process representations and later transitioned to product representations, which stemmed from recent efforts to include new technological advancements and building systems into the schema. Comparatively, the infrastructure domain initially included cases focusing on product representations and later transitioned to processes. The initial efforts seem to have catalyzed the inclusion of entities and properties exclusively for infrastructure products, especially in the latter versions of the IFC. These advancements, in turn, were followed by extensions for defining concepts and processes specific to select sectors in the domain. The approaches for implementing the extensions varied depending on the extensions' purpose and correspondingly utilizing either STEP or BIM authoring tools. Many cases were limited to documenting their extension designs using EXPRESS-X or EXPRESS-G without the implementation of actual software. Validation methods differed depending on the extension approach used, as follows: entity and property extension cases were limited to demonstrating interoperability, while property extensions went a step further by demonstrating the utility of the extensions post-export into other software applications. Based on these findings, strategies for future extensions were outlined. In the architectural domain, a sustained focus on process representation is recommended, in line with the growing trend of adopting 'entity and property extensions' to incorporate emerging technologies and advanced building systems. Moreover, these extensions should be subjected to rigorous 'utility-centric' validation processes to validate their incorporation in forthcoming schema updates. For the infrastructure domain, it is suggested to shift focus towards underrepresented sectors, such as dams and airport facilities, which require specific definitions within the schema. Conversely, for well-established sectors, such as bridges and tunnels, it is recommended to enhance process representations, drawing insights from the historical developments in the architectural field. Figure 10: Modularization concept of IFC5 [97] (reprinted, with permission from [PERSON] et al. 'Future of the Industry Foundation Classes: towards IFC 5’, _Proc. of the Conference CIB W78_, Luxembourg, 2021, 123–137.). This study concentrated on structuring and classifying IFC schema extension cases to derive insights and trends from which future directions in this research area could be derived. Thus, in addition to the findings, the methodology also provides a template for conducting similar analyses and formulating strategies for future works in IFC schema extensions. Together with the initiatives for modernizing the IFC, the findings are hoped to expedite the development of BIM-based collaboration frameworks in the AEC/FM industry. Supervision, B.K. ; Writing--original draft, Y.Y., S.K. and H.J.