As you've seen, HTML is a defined set of markup tags, whereas XML is a set of rules for creating markup tags. So what does that mean? Compare the two variations of markup presented earlier. The files contain the same information. However, to authors the tag names themselves offer additional detail about the information. The tag names become metadata.
For occasions where additional information is required to describe content, attributes can be used to further define metadata. An attribute is a name and a value that can be associated with a tag. For example, a common use for metadata in a reuse environment is to indicate who the intended audience is for a specific piece of information. Consider the procedural example. Using attributes, you can identify the audience for each specific step, option, or even word. Listing 1 shows the same procedure but with a step added. The step is tagged like all the other steps, with the addition of an attribute (product="extended"). This attribute functions as metadata in that it indicates that this step is applicable only in the extended version of the product.
Listing 1XML with attribute metadata
<procedure> <title>Logging On to AccSoft </title> <paragraph>The first time you click on a component in AccSoft you are required to log on to the system before you can complete any tasks. </paragraph> <intro>To log on to AccSoft:</intro> <procedure_steps> <step>Double-click the AccSoft application.</step> <step>Select Accounts Payable from the Explorer.</step> <step>Type your USERID into the Name field.</step> <step>Type your password into the Password field.</step> <step product="extended">Select the customer to update.</step> <step>Click the OK button to log on to AccSoft.</step> </procedure_steps> <exercise>Log onto the training database using the USERID and password provided by your course facilitator. </exercise> <note>If you do not know your USERID or Password, consult your System Administrator. </note> <warning>This database contains personal information about our clients. Do not let anyone else use your password at any time. </warning> </procedure>
Using traditional document and content management systems, authors add metadata through a selection window when they check in a file. The best CMS products include search tools that enable users to search by metadata. But the metadata is only associated with the file; it is not part of the file. You can't email the file to someone and have the metadata go along because the metadata is part of the CMS data. In XML, the metadata travels with the XML file. It is entered as the file is authored or updated. It remains part of the content and can be easily searched.