Use of the OnDemand attribute is no longer recommended. Instead, you should consider using Features and Recipes to group content into independent addable and removable groups. Features and Recipes still allow you to specify what content installs by default, just like the OnDemand attribute.
OnDemand attribute to a chunk in your layout.xml file as shown in the following example.
OnDemand attribute must also include custom tag specifiers so that runtime APIs can reference the associated chunk by tag.
Titles can uninstall OnDemand content by calling the XPackageUninstallChunks method.