In shortA data contract is a written agreement between source and consumer on the fields, types, units, time and quality rules and owner of a data flow.
Most integration problems come not from the connection but from differences in interpretation: is the temperature °C or °F, when does the counter reset, is time local or UTC? A data contract answers these questions in advance.
A good contract contains at least: equipment identifier, source timestamp, unit, quality field, behaviour for missing and bad-quality values, version and change process. Automation, production and IT teams should write it together during the pilot.
In OPC Router projects
In an OPC Router workflow, transformation steps are the technical counterpart of the data contract; the clearer the contract, the simpler the workflow.