Here's the ACBL's alert chart: http://web2.acbl.org/documentLibrary/play/AlertChart.pdf

Here's the ACBL's alert procedures: http://web2.acbl.org/documentLibrary/play/AlertProcedures.pdf

I sort of understand the differences, but then again I sort of don't.

Is an announcement a type of alert?  Why do we announce some things, but alert others?

Why does a 15-17 point 1NT opener need to be announced, as opposed to 16-18 or 12-14 or whatever range you use?

Why isn't a transfer over NT considered an alert (it's an announcement)?

Why isn't Unusual 2NT or most cuebids alertable?

I just don't really understand how the ACBL decides it's going to treat each situation.