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use="a-class-name"
use="${EL_returns_a_class_or_a_class_name}"
use="${a_component}" (since 3.6.0)

It specifies a class to create a component instead of the default one. In the following example, MyWindow is used instead of the default class, Window.

 <window use="MyWindow"/>

If an EL expression is used, it can return a class name, a class instance, or a component instance. Notice that, if the expression returns a component, the component shall not belong to any page.