## 4 Mark-up strategy
* The information pertaining to one viewpoint can be put in a separate markup file; and multiple independent markup files can be safely applied to the same object. Thus, it allows context-specific information to be manipulated into different viewpoints, freely tailored to a reduced version for reasons of security/IP (Intellectual Property) and the requirements of different target users, and re-organized for various purposes and applications. The major disadvantage of'stand-off' markup is how to implement it, specifically how to get robust, persistent references back to the object and how to link from the object to the markup? Table 2 summarizes advantages and disadvantages of the two strategies. Although more work is needed on its implementation, comparing to 'in-line' markup, the application of the'stand-off' markup has shown more great potential in engineering design, particularly on the trend to management of product information throughout its lifecycle. In general, \begin{table}
\begin{tabular}{|l|l|l|l|} \hline
**Markup strategy** & \multicolumn{2}{c|}{**Advantages**} & \multicolumn{2}{c|}{**Disadvantages**} \\ \hline 'in-line’ & \(\bullet\) & Easily implementation & \(\bullet\) & Document changed \\  & \(\bullet\) & Wide applications & \(\bullet\) & Difficulty for multiple independent \\  & & & sets of markup \\ \hline ‘stand-off’ & \(\bullet\) & Non-change of representation & \(\bullet\) & Difficulty of implementation \\  & & method used for the original object & \(\bullet\) & Problem of persistent references \\  & \(\bullet\) & Support of multiple independent sets & \(\bullet\) & Lack of robust maintenance method \\  & & of markup & & of references \\  & \(\bullet\) & Support of progressively information & \\  & & update & \\  & \(\bullet\) & Capability of re-organization of & \\  & & markup information for different & \\  & & purposes and applications & \\ \hline \end{tabular}
\end{table} 
Table 2: Advantages and disadvantages of markup strategies. product information encompasses a vast heterogeneous range of information, required and produced by many different individuals and groups who interact with the product throughout its lifecycle. Each of these actors requires different subsets of the product information and different structuring of the information. In addition, 3D digital product models (_i.e._ CAD models) have already dominated in industry design. The structure of CAD models (e.g. B-rep) is designed for representing 3D geometrical information, but has only limited capabilities for storing markups natively. Meanwhile, a number of geometrical representation methods make it uneconomical or even impossible to insert various types of markups using the 'in-line' method. The'stand-off' approach, however, allows markup information to be stored in separate documents and linked back to the model using references or pointers, and therefore it actually extends the product models to additional engineering and non-engineering information not currently supported by the established formats. This will be further discussed in the next section. Figure 2 shows the comparison of 'in-line' and'stand-off' markup methods. In practice, markup is usually done manually, semi-automatically or automatically according to how much human effort is involved. Manual mark-up is the most popular, most accurate (with human interpretation) but most labour-intensive. Automatic mark-up reduces manual intervention and gives a more integral representation of the documents. There are limited publications which are concerned with automatic and hybrid mark-up. For example [45-47], explored automatic mark-up with different methods and targeting on different types of documents. In [45], the authors reported on the design and implementation of a system which automates the process of capturing structured documents from the optically recognised form of printed materials to elements like words, sentences, title, authors, _etc._ but not for logical content elements.