UI Logging Level Options vs. Agent Logging Options

On the Stat Configuration page, you are presented with logging choices:

UI Logging Level :       [drop-down menu]

Agent Logging Level :  [drop-down menu]

I haven't found much about this in the Admin Guide, the Install Guide or the Upgrade Guide.  So I ask:

* Under what circumstance would I set the UI Logging level to DEBUG instead of using the Agent Logging Level?

* While I have used the Agent Log options, I have never used the UI Logging Level options.  Can someone share what they can be used for?

Thank you!