A predefined add-in ID or COM GUID. Reference IDs don't have to come from your add-in, the ID can be a reference to a type declared in another add-in that will also be loaded at run-time. Reference IDs can also be a GUID representing a registered COM type. For example, when defining a toolbar you may list custom buttons from your add-in as well as some COM commands/tools such as the ESRI built-in ones like the Pan Tool.

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

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