Managed Network Objects - Advanced Alerts

I'm not sure I understand what you're trying to do, but I'm going to assume that you want to alert on only some network devices.  In Advanced Alerts, the best way to do this is to assign a custom property to your devices.  Then you create an alert that triggers only with the condition of interest is true and the custom property has a certain value.  For example:

Alert if ALL of the following are true:

Alert me if a Node is Down

Custom Property = "foo"