The beauty of OPC is that once the OPC Server has information, any OPC Client can easily retrieve it without having to worry about the specific PLC format. However, configuring an OPC Server to communicate with a PLC takes some work. A common issue is what to do about data sources (PLCs or DCSs) that have a specific structure for the data. With OPC Data Access (DA), the most common mechanism to describe the data is to simply use a common naming convention. Thus, "FIC101.PV" provides the Process Variable, "FIC101.SP" provides the Setpoint, etc.
Some OPC Servers are aware of structures in the device to which they are supposed to connect. In this case, the OPC Server can configure itself automatically for the scenario above. Other OPC Servers do not have a predetermined structure and require Integrators to set these up repeatedly for each project. The ability for an OPC Server to automatically configure itself to a PLC, or do so with minimal integrator interaction is a key differentiator between OPC Servers. The OPC Training Institute encourages you to find out how a server handles configuration at the time project requirements are set.