IEC 61850 seems to a major standard for electrical power distribution, but I don't play there so you'll have to look into that on your own.
In the process world, HMI/SCADA/DCS are usually OPC clients.
The rule is that OPC client talks to one or more OPC servers and an OPC server talks to one or more clients. The OPC server connects to the hardware (with a low level driver) and handles the data tag values for transfer to/from the OPC client.
When an OPC client connects to several OPC servers, it is called OPC aggregation.
When an OPC client connects to an OPC server over a network, it can be done with OPC tunnelling.
When an OPC server connects to another OPC server to share data, it is known as OPC bridging.
I suspect that some HMI/SCADA/OPC packages can operate independently as an OPC client and an OPC server. It appears to me that a work-around for OPC-client-to-OPC-client communication is to go through a common server or tunnel OPC server to server.