The OPC UA "Aggregating Server" pattern consists of exposing, through a single OPC UA server, data coming from multiple sources (Modbus PLCs, BACnet devices, SQL databases), so as to simplify integration on the SCADA/MES side.

An Aggregating Server is a single front-facing OPC UA server with backend connectors to heterogeneous sources: Modbus PLCs, BACnet devices, Siemens S7 controllers, SQL databases, REST APIs and so on. Each source is mapped into a hierarchical branch of the OPC UA Address Space, so a SCADA or MES client sees one unified semantic tree instead of several incompatible drivers. This pattern is the de-facto standard for modern data aggregation in Industrie 4.0 and is implemented by products such as Kepware KEPServerEX, Matrikon and the Unified Automation OPC UA Gateway.

Treat the aggregating server's Address Space as a contract for downstream clients: design it to be vendor-neutral so you can swap a Modbus PLC for an OPC UA-native PLC later without breaking a single SCADA tag binding.