# A Systematic Review of the Trends and Advances in IFC Schema Extensions for BIM Interoperability
## 5 Evaluation of IFC Schema Extension Cases
### Analysis of Extension Cases by Product and Process Representations
#### 5.3.3 Extension Trends Comparison between Architecture and Infrastructure Domain  
Figure 8 shows the timeline for the number of extension cases performed for the architecture domain between 2001 and 2022. The red and blue lines show the trends for product and process representations, respectively. The trend lines show that most of the extensions comprised process representations throughout the period. However, this trend started to change starting in 2018, and there were more extensions for product representations in 2022. The trendlines reinforce the evaluations discussed in Section 5.3.1. That is, as the initial IFC schema already had comprehensively defined elements in the building domain, most extension efforts focused on representing concepts and processes required for engineering analysis and project management procedures. The increase in product representations near the end of the period represents the recent trend of initiatives to integrate information from new technologies (e.g., IoT, point cloud data, etc.) and building systems into the IFC schema.  
Figure 9 shows the corresponding timeline for the infrastructure domain. The trend lines show that extensions initially focused on product representations, which were later followed by a rise in process representation cases. Although product representation cases remained steady, there was a notable uptick in the cases focusing on process representations. Specifically, from 2019 onwards, the number of process representation cases began to overtake those of product representation cases. As discussed in Section 3.1, this shift can be primarily attributed to version 4.0 of the IFC schema, which introduced base entities for infrastructure, which were provided in part by the preceding extension studies in product representations. Such trends are similar to those observed in the architectural domain. That  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Extension** & **Sector** & **Product-** & **Number of** & **Process** & **Number of** & **Total** \\
**Approach** & **Sector** & **Oriented** & **Cases** & **Oriented** & **Cases** & **Total** \\ \hline \multirow{4}{*}{Entity + Property} & Tunnel & [57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67] & 8 & [65, 66, 67] & 3 & 11 \\  & Bridge & [68, 69, 70, 60, 61, 62, 63, 64, 65, 66, 67] & 3 & [30, 72, 73, 74, 75, 76, 77] & 7 & 10 \\  & Road & [78, 79, 80, 81, 82] & 3 & [74, 81, 82] & 3 & 6 \\  & Rail & [84, 86, 87] & 3 & [76, 77, 85, 88] & 3 & 6 \\  & Other & [18, 19, 92, 93, 96] & 5 & [14, 89, 90, 94, 95] & 5 & 10 \\ \hline \multirow{4}{*}{Property} & Tunnel & - & - & - & - & - \\  & Bridge & - & - & [71] & 1 & 1 \\  & Road & - & - & [83] & 1 & 1 \\  & Rail & - & - & - & - \\  & Other & - & - & [91] & 1 & 1 \\ Total & - & 22 & - & 24 & 46 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Extension cases by approach type in infrastructure domain.  
is, as the core of entities for products matured, there was an escalated interest in representing process information, encompassing areas such as structural analysis, inspections, O&M, and design details.