A Companion Specification is the specification of a domain-specific Information Model based on OPC UA. Typically, a Companion Specification is defined in a working group of the OPC Foundation or a joint working group of the OPC Foundation with other organizations like the VDMA. Potentially, a Companion Specification can be defined without explicit cooperation with the OPC Foundation, for example by a standardization organization or a company.

A Companion Specification consists of a document defining the Information Model and a UaNodeSet as a machine-readable file used by implementations of the Companion Specification.