From Joomla! Documentation
This Wiki aims to document as many (non-deprecated) versions of Joomla! (both the CMS and the Platform) as possible so as to provide relevant information for all users. Therefore it is important to indicate the version(s) each piece of information (e.g. page, paragraph) applies to clearly and uniformly. This page summarizes and explains the methods for organizing and tagging Wiki content with specific versions.
Icons are a quick and familiar way for users to recognize whether a page applies to their version of Joomla or reinforce it if the page is in a version specific namespace. Icons can currently be added using the JVer template, which can be used for placing CMS version icons, Platform version icons and a generic Platform icon. For example,
Placing pages in categories is essential to the organisation of our documentation. Just as Joomla requires categories as a more efficient method of organisation, so does our documentation wiki. You can place a page in more than one category if it applies to more than one type of category.
The Joomla! Documentation Wiki articles are placed in a version appropriate or descriptive namespace. This helps identify the text of the article with the version of Joomla it refers too. For example, the top of this page should show JDOC in a blue block. The title of the article is shown in the URL address bar as "JDOC:Joomla!_versions" with just the title below the blue block showing the namespace. JDOC, signifies it is the "project" or wiki's namespace for documentation articles. These articles would be for information, procedures, styles, instructions, etc. for using Joomla Documentation's website. If this article was for Joomla 2.5, you would see a J2.5 in the blue box and likewise the article's name would really be "J2.5:Joomla! versions."
Some pages apply to more than one version or some versions of Joomla!. These may or may not be version specific and apply across multiple or all versions of Joomla. In these cases, no prefix should be used. For example, the article's name is "Using TinyMce as My Editor" which could apply to all Joomla versions. So you would not have a J2.5 or J3.x as a prefix and no blue box will appear.
For further reading and to find out about all the namespaces used on this wiki, please see Namespaces.
One of the nice features of using a namespace is the ability to set parameters for all the pages in a namespace. Styling or notices can easily be added and at the same time provide different types of feedback to search engines as to the relevance of a page in a namespace. For example, currently all articles in the J1.5 namespace are archived. While the content is fully intact, it is not being indexed as aggressively as the current versions of Joomla.