With OPC UA being the mandated machine interface by more and more end users the quality of OPC UA enabled applications becomes crucial to the success of the end users in their digitalization and IIOT projects. The OPC Foundation certification program provides testing for OPC UA core technologies and the Information Models of the Companion Specifications to provide confidence to the end users in the supplied systems and an easy integration process. Examples of companion specifications which are already testable and success stories where end users benefit from applied companion specifications will demonstrate the advantages.