## 6 Summary and discussion  
Digital annotation has developed rapidly during the last two decades. Nowadays, annotation has been widely used in different digital items, including text documents, structured data (e.g. databases and tables), 2D images, 3D models and multimedia (e.g. video and audio). This paper has presented a contemporary review of the latest research into the area of annotation and its subtype, markup related to engineering design. It has shown that:  
1. Annotation and markup have shown their advantages in various applications. XML, as one of most common markup languages, has been widely applied due to its characteristics, especially its extensibility, representation of semi-structure information, and support of re-organisation and multi-viewpoints. As shown in the review, the powers of XML have led to a large number of XML-based markup languages founded on various scenarios and \begin{table}
\begin{tabular}{|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|} \hline Work by [PERSON] and [PERSON], 1998 [58] & In-line & N/A & Product model & Multiple viewpoints & Unclear \\ \hline CAD/CAM (e.g. AutoCAD 2008, SolidEdge V19, Pro/ENGINEER, NX5, CATIA & & & & \\ \(\
abla\)SR13), Microstation [59-64] & & & & \\ \hline LIMMA by [PERSON] _et al._ 2009 [14] & Stand-off & Manual & CAD models & Multiple viewpoints & XML \\ \hline EPM by [PERSON] _et al._ 2009 [1] & Stand-off & Manual & CAD models & Information organization and retrieval in PLM & XML \\ \hline TaDiMa by [PERSON] _et al._ 2009 [65] & In-line / stand-off & Automatic & Digital Master & Marketing and design review & HTML \\ \hline Work by [PERSON] and [PERSON], 2006 [67] & In-line & N/A & X3D, semantic web & Navigational support & X3D/RDF/XML \\ \hline Work by [PERSON] _et al._ 2009 [68] & Stand-off & Manual & CAD models/ Lightweight representations & Management of lifecycle product information & XML \\ \hline Vannotea by [PERSON] _et al._ 2006 [69] & Stand-off & Manual & Images, video, 3D object, audio & Communication in collaboration & XML \\ \hline Work by [PERSON] _et al._ 2009 [70] & In-line & Manual & 3D object & Communication and knowledge management & Unclear management \\ \hline Work by [PERSON] and [PERSON], 2009 [71] & Stand-off & Manual & VRML & Communication in collaboration & VRML \\ \hline Work by [PERSON] _et al._ 2009 [72] & Stand-off & Manual / Semi-automatic & CAD models & Support of design optimisation & XML \\ \hline Work by [PERSON] _et al._ 2009 [73] & In-line & N/A & X3D & Multidisciplinary engineering environments & X3D/OWL environments \\ \hline \end{tabular}
\end{table} 
Table 3: cont. applications, which have resulted in the problem of information interoperability when considered for future usage. 2. There are two basic types of markup methods: 'in-line' and'stand-off'. The 'in-line' method can be easily implemented using a text editor or a simple scripting language, and therefore it has been widely adopted, especially in document management. However, with high requirements to support multi-viewpoints and the introduction of 3D annotation,'stand-off' method starts to show its potential. Although some technical issues like persistent references and maintenance of changes, have not yet been adequately solved, which hinders its applications in industry. 3. It is inspiring to see that annotations have played a key role in product development. The work on information retrieval comes not only from the engineering community, but also from computer science and IT industry. The current achievements encourage researchers to think about extending the application from document to 3D models and multimedia. Compared with document retrieval, the researches on 3D annotation started a little later, but the latest results, such as exploration of'stand-off' annotation on 3D models, integration with PLM requirements and datasets, and applications in collaboration environment, have already shown their considerable impacts for industrial solutions.