A "network" is any collection of server machines that are physically connected and/or can see each other.
Now consider some subset of those servers that are configured to coordinate with each other. One might be an authentication server, another a database server, and two are application servers that rely on the first two for SSO, application data and maybe shared inter-application data.
To the end-user there is one "thing" offering multiple services. What single word or phrase is the name of this thing?
For example, "The [coordinated subnet] provides the chat service."