Health check propagation

Documentation about health checks states the following:

To propagate status changes (e.g. all servers of this service are down) upwards, HealthCheck must also be enabled on the parent(s) of this service.

Can you please explain this in more detail? What does it mean that health check must be enabled on the parents of the service? Can you provide an example configuration where all servers of one service are down so parent service routes the request to another service?