This section defines the ReferenceTypes that are inherent to the present companion specification. Figure 27 describes informally the hierarchy of these Reference Types. OPC UA Reference Types are defined in OPC 10000-3.

Figure 27 – Reference Type Hierarchy

The defined non-Hierarchical References are essential to the semantics of the model. When the underlying system changes, the underlying system shall update the references consequently in the OPC UA model.

For instance, let us consider a filter combiner with machine modules A, B, C where the product flows from A to B to C. In this scenario, the non-hierarchical reference FlowsTo points from A to B, from B to C. If the machine module B is removed and replaced by machine module D, then the underlying system shall remove the non-hierarchical references FlowsTo A to B, B to C and create non-hierarchical FlowsTo references A to D, D to C.