The trigger in your first example is the one to use "Volume Percent Used". The 2 Disk Utils you see are custom properties that someone has created on your system. They could be used for a trigger where you write a generic trigger for all volumes but you only wish to be alerting on some. So on those volumes you change the custom property to yes and in your trigger you add in the custom property to yes. That way it will only trigger on those volumes that have the custom property set to yes.
I hope that makes sense.