The default library for all the classes in the add-in. If all your types originate in the same library, use this attribute to set it as the default.

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

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