## 4 Mark-up strategy  
The conventional method of markup of a document is 'in-line' with the text of the documents - _i.e._ text markup labels merged into a text document. From a computer system's point of view, the markups change a document from a long sequence of strings to a block of information with semantic meaning. The 'in-line' markup method is easily accomplished using a text editor or a simple scripting language, and therefore it has become one of the most commonly used methods. However, there are some disadvantages of the 'in-line' markup method. For example, as the method inserts markup directly into the text of the document, it actually changes the document. As the markup labels are embedded in the document, it is difficult to place multiple independent sets of markup in the same document without them interfering with each other. \begin{table}
\begin{tabular}{l l|l|l|l|l} \hline eXtensible Markup Language & SGML & W3C & 1996 & Arbitrary data structures \\ (XML) & & & & \\ \hline Dimensional & Mark-up & XML & NIST & – & CAPP/CAM/CNC \\ Language (DML) & & & & \\ \hline Product Information & Mark-up & XML & [PERSON] _et al._ & 2005 & PLM \\ Language (PIML) & & & & \\ \hline eXtensible Rule & Mark-up & XML & [PERSON] and Lee & 1998 & Web information \\ Language(XRML) & & & & \\ \hline Motion & Capture & Mark-up & XML & [PERSON] and Lee & 2003 & Motion capture data \\ Language (MCML) & & & & \\ \hline Scalable & Vector & Graphics & XML & W3C & 2001 & 2D graphics \\ (SVG) & & & & \\ \hline Vector & Markup & XML & Autodesk, Hewlett- & 1998 & Vector information \\ Language(VML) & & & Packard, Macromedia, & & together with additional \\ \hline Precision Graphics Mark-up & XML & Adobe Systems, IBM, & & \\ Language(PGML) & & & Netscape, Sun & – & Vector graphics \\ \hline X3D (eXtensible 3D) & XML/ & W3C/ISO & – & 3D graphics \\  & VRML & & & \\ \hline
3D XML & XML & Dassault Systemes & – & 3D data \\ \hline PLM XML & XML & UGS. & – & PLM information \\ \hline Description & Framework & & W3C & 1999 & Web resources \\ (RDF) & & & & \\ \hline Web Ontology Language (OWL) & RDF/ & W3C & 2002 & Web ontology language \\ \hline Commerce XML (cXML) & XML & more than 40 companies & 1999 & e-commerce \\ \hline Simple Object Access & XML & W3C/ Microsoft, IBM, & & 2003 & Web service \\ Protocol (SOAP) & & & UserLand Software) & & \\ \hline Web Service Description & XML & W3C /IBM, Microsoft, & 2000 & Web services \\ Language (WSDL) & & & Ariba) & & \\ \hline Remote Procedure Calls & XML & [PERSON] of & 1998 & Internet \\ (XML-RPC) & & & Microsoft & & \\ \hline \end{tabular}
\end{table} 
Table 1: _cont._To overcome this, an alternative method called'stand-off' markup [41, 42] is introduced. Different from 'in-line' markup, the content in the'stand-off' markup is stored in separate external documents utilizing a system of references or pointers to which element the markup refers. The work of [PERSON] _et al._[14], [PERSON] [43] and [PERSON] [44], has shown that the'stand-off' markup method has many advantages over \"in-line\" markup, including:  
* It allows original digital object (_i.e._ including both documents and various digital model) progressively to expand to include extra information (e.g. semantic context and rationale behind) and metadata without changing the representation method used for the original object. Thus, it provides a good tool to support the continuous update of information throughout a product lifecycle. * Many layers of markup can be associated with the same object, allowing different viewpoints to be associated with, both concurrent viewpoints (e.g. cooperators or partners during the design phase) and successive (e.g. technology requirements from machining engineers or maintenance information from in-service phase). That is, with'stand-off' markup, information from various viewpoints with different structures or formats can be carried, while the original object itself however, remains 'light' and can be passed with only the information necessary for a specific user or purpose.