## 6 Discussion
### Innovativeness of Design Knowledge Annotation  
The process of design knowledge annotation realizes the integration and fusion of 3 DAM and design knowledge, and forms 3 DIM. The process of design knowledge annotation proposed in this paper is similar to that of EAM construction in some aspects, for example: (1) realize the association and display of design knowledge and product information through the application program interface provided by specific CAD software; (2) the design knowledge is recorded by general XML file; (3) through the secondary development of CAD software, the corresponding user interface is formed. However, 3 DIM has a wide range of design knowledge types than EAM, and the object of design knowledge annotation extends from geometric information (mainly features) in EAM to geometric and technical information. Therefore, the proposed design knowledge annotation method needs to consider more product information integration and mapping problems. Table 2 shows the comparison between the proposed method and the annotation method of EAM. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Comparison Point** & **EAM** & **3 DIM** \\ \hline \multirow{6}{*}{Overall comparison} & Object of & \multirow{2}{*}{Geometric information} & Geometric and \\  & knowledge annotation & & technical information \\ \cline{2-4}  & & & \\ \cline{2-4}  & Content of & & & Design intent (including geometric \\  & knowledge annotation & & design intent and technical design \\  & & & intent), design justification, design \\ \cline{2-4}  & The existing form of & & & decision and design option \\ \cline{2-4}  & design knowledge & & & \\ \cline{2-4}  & Uniqueness of & & & \\ \cline{2-4}  & annotated objects & & & \\ \cline{2-4}  & Granularity of & & & \\ \cline{2-4}  & knowledge annotation & & & \\ \hline \multirow{6}{*}{Comparison of XML file structure} & Correspondence between & \multirow{2}{*}{One XML file corresponds to} & \multirow{2}{*}{One XML file corresponds to one} \\  & XML file and product model & & multiple product models & product model \\ \cline{2-4}  & Basic structure unit of & & & \\ \cline{2-4}  & XML file & & & \\ \cline{2-4}  & & & & \\ \hline \multirow{6}{*}{Comparison of  annotation process} & Existence checking and & \multirow{2}{*}{Not mentioned} & \multirow{2}{*}{The design knowledge for} \\  & initialization of XML file & & & \\ \cline{2-4}  & & & & \\ \cline{1-1} \cline{2-4}  & & & & \\ \cline{1-1} \cline{2-4}  & Comparison of & & & \\ \cline{1-1} \cline{2-4}  & & & & \\ \cline{1-1} \cline{2-4}  & The display way of & \multirow{2}{*}{Design knowledge is  displayed in the newly  plane; The design knowledge for} \\  & design knowledge & created annotation plane in & \multirow{2}{*}{technical information is directly  displayed in the annotation plane} \\ \cline{1-1} \cline{2-4}  & the form of association group & & & \\ \cline{1-1} \cline{2-4}  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The comparison of design knowledge annotation method. It can be seen from the table that the design knowledge annotation method proposed in this paper has the following advantages compared with EAM annotation method:  
1. 3 DIM has more extensive connotation and application scenarios. Compared with EAM, the annotation object of 3 DIM model is not only limited to product geometric information, but also includes technical information. Moreover, the content of annotation knowledge is more extensive, including design intent, design justification, design decision and design option, which provides richer knowledge for the product model and comprehensively improves the quality of product model. At the same time, the granularity of knowledge annotation changes from feature level annotation to more fine-grained product information level annotation, which makes the object of design knowledge more specific.