monitoring interval for a device category vs. monitoring interval for a particular monitor (e.g. disk utilization)

monitoring interval for a device category vs. monitoring interval for a particular monitor (e.g. disk utilization)

What is the relationship between monitoring interval for a device category (Admin->Monitoring Intervals)
 vs.  
monitoring interval for a particular monitor (e.g. disk utilization)?

Every monitor (e.g. disk utilization) has a required interval field.  Does that make the device category monitoring interval unnecessary?

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