## 2 Annotation and markup  
Annotation can be simply understood as the act of adding additional information for various purposes, such as commentary, viewpoint interpretation or extra description. Annotation has been a part of engineering practice to aid communication for a long time, for example, engineers discuss the product face-to-face through annotating a design drawing, or sending an annotated drawing of a product to colleagues (an example is shown in Figure 1). Markup may have different definitionsaccording to its application domains, such as the publishing industry or in the world of computer science. In this paper, markup is regarded as a subtype of annotation and is defined as a formally structured annotation for a purpose, normally to allow some kind of manipulation of the information entity [14]. Over last two decades, many mark-up methods have been developed and these can be classified into six basic categories [15-17]:  
1. _Punctuational mark-up_: where word, phrase, and sentence boundaries are identified by spaces, commas, full stops, and other punctuation characters inserted into the text.
2. _Presentational mark-up_: where the visual form of the document is specified directly.
3. _Procedural mark-up_: in which presentational instructions (or commands) for some particular processing system are embedded in the text.
4. _Descriptive mark-up_: the author identifies the element types as tokens, as often found in applications of SGML and XML, which approach documents as structured objects containing semantically interpretable parts.
5. _Referential mark-up_: refers to entities external to the document and is replaced by those entities during processing.
6. _Meta-mark-up_: provides a facility for controlling the interpretation of mark-up and for extending the vocabulary of descriptive mark-up languages (e.g. macros).  
To date, digital information has predominately been stored in different structures and formats depending on which specific software being used. However, it is normal that such information needs to be translated into different formats (e.g. Microsoft word format, HTML, PDF) for purposes such as presentation and accessibility in different contexts. A typical example is graphs of experimental results embedded in Microsoft Word document. Such traditional methods succeed in visualization of graphs,  
Figure 1: An example of annotation  
but fail by missing other information, like how the results were produced and the raw data which they represent. In terms of solving this issue, descriptive markup is able to show potential benefits. Firstly, the descriptive markup allows users to focus on both the structure and content of document so it has greater portability. Secondly, descriptive markup separates how data is stored and how it is used and therefore can potentially provide any additional information required when transforming the object to different formats for the different purposes or when to be re-processed or re-organized for different applications and uses.  
To allow markup to be implemented and to serve applications in engineering design, it is essential to have adequate understanding of markup languages and strategies. These will be discussed in the next two sections.