In XSI, an Add-on refers to a particular group of files that act together to add some functionality to the XSI package. XSI ships with several Addons, including the Syflex tool and many SDK examples. The Add-on can contain many different types of files, including Self-installed Plugins, Shaders, Operators, Toolbars, Netview content, and so on.
Each Add-on has its own directory underneath the "Addons" directory of the User, Workgroup or Factory locations. (In previous versions of XSI Addons were often stored without this subdirectory, but this meant that it was difficult to tell which files belonged to which Addon).
The primary way to distribute an XSI add-on is by the .xsiaddon file format, which is an XML file. .xsiaddon files are easy to install and uninstall directly within XSI, and often it is not necessary to restart XSI to start using the contents of an .xsiaddon. (See Add-on Packages (http://softimage.wiki.softimage.com/sdkdocs/cus_addons_AddonPackages.htm))
Addons can also be stored inside "zip" files or installed using a professional installation tool like InstallShield.