Clustering allows a single listener to distribute requests to dispatchers on multiple machines. It also allows a single dispatcher to distribute requests to SEPs on multiple machines. When scaling out, the listener, dispatcher and SEP can operate on different machines.
For the event monitor service:
The cross-region database and region databases must exist in the same database server instance. That is, a region database operating in one database server instance cannot work with a cross-region database operating in a different database server instance.
A database server instance can exist on the same machine as the listener, dispatcher or SEP processes, although in a live environment, this can have performance and security implications.