Examples: github.internal.companyname.com or jira.corp.org or jenkins-ci.internal-finance.acme-corp.com or grafana.monitoring.initech.io or confluence.prod.internal.companyx.com etc

These, if you don't know the host, you will not be able to hit the backend service. But if you know, you can start exploiting it, either by lack of auth, or by trying to exploit the software itself