This tool is not very useful to call directly. It is however used by NetworkManager-wait-online.service with --wait-for-startup argument. This is used to delay the service and indirectly network-online.target, until networking is up. Don't order your own systemd services after NetworkManager-wait-online.service directly. Instead if necessary, order your services after network-online.target. Even better is to have your services react to network changes dynamically and don't order them with respect to network-online.target at all.
By default, connections have the ipv4.may-fail and ipv6.may-fail properties set to yes; this means that NetworkManager waits for one of the two address families to complete configuration before considering the connection activated. If you need a specific address family configured before network-online.target is reached, set the corresponding may-fail property to no.
-h | --help
-q | --quiet
-s | --wait-for-startup
There are various ways to affect when startup complete is reached. For example, by setting a connection profile to autoconnect, such a profile possibly will activate during startup and thus delay startup complete being reached. Also, a profile is considered ready when it fully reached the logical connected state in NetworkManager. That means, properties like ipv4.may-fail and ipv6.may-fail affect whether a certain address family is required. Also, the connection property connection.wait-device-timeout affects whether to wait for the driver to detect a certain device. Generally, a failure of NetworkManager-wait-online.service indicates a configuration error, where NetworkManager won't be able to reach the desired connectivity state during startup. An example for that are bridge or bond master profiles, that get autoconnected but without activating any slaves. Such master devices hang in activating state indefinitely, and cause NetworkManager-wait-online.service to fail.
-t | --timeout seconds
-x | --exit
nm-online exits with status 0 if it succeeds, a value greater than 0 is returned if an error occurs.