Hi Henk, from the docs https://www.dynatrace.com/support/help/shortlink/services-usage
Calls to unmonitored hosts: These represent calls that resolve to a private (internal) IP address and are usually under the responsibility of your organization. Therefore dedicated monitoring of these dependencies with OneAgent, or via Custom device monitoring, is recommended to gain visibility into these parts of your environment.
So it does not depend on the type of service.
You can view requests to unmonitored hosts as well as external services since the Oneagent lives on your host. Lets say your Server.... Server 1 has a oneagent and reaches out to a 3rd party banking app called Banker. You will see the request being generated from Server 1 to Banker. You should also be able to see incoming requests too, bu that information will be limited.