This field (optional), which appears under the ‘Common settings’ tab of a Service definition, is used to identify the hostgroup(s) that the service “runs” on or is associated with. Contrary to the Host groups directive in a host definition, this directive may NOT be used as an alternative to (or in addition to) using the “Members” directive in “Host groups”.
For better manageability, it is recommended to use the “Hosts” field instead of the “Host groups” field in the service definition to associate the service to one or more hosts.
One of the reasons for using this approach is that if you use the “Host groups” field to associate the service, then association is made for all hosts member of the “Host groups” field. This could pause problems because if the service is not available on one of those hosts, then a false positive alert will be generated for a service that doesn’t exist on the host(s).
Note: If you like, you can select one or more Host groups and one or more Hosts for the same service. This option work as well.