What requirements to get to the interoperability?

What requirements to get to the interoperability between IEDs and between systems based on IEC61850?
This is one of the main issues and the goal of the IEC61850 standard.
IEC 61850 was developed to allow integration of devices from different manufacturers based on the concept of interoperability and support of different philosophies and ability for free allocation of functions and has a DATA MODEL that can be accessed. So, the standard creates the possibility of interoperability, but does not create its direct application.
Interoperability is the ability for IED’s from one or several manufacturer to exchange information and use the information for the their own functions.
What requirements must be met to implement interoperability between IEDs or systems? What are the main points that must be specified to promote interoperability?
What should be specified to be interoperable?

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