The default namespace for all the classes in the add-in. Use this attribute instead of specifying the full class name (namespace.className) on each of your types.

Namespace: http://schemas.esri.com/Desktop/AddInsSchema: DesktopAddIns_ElementOnly.xsd

<xs:attribute name="namespace" type="xs:string" use="optional" xmlns:xs="http://www.w3.org/2001/XMLSchema"></xs:attribute>