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We use the Warning/Caution/Note system in our literature too (we're an ISO certified company). We put more emphasis on the Warnings and Cautions, however, which are important from a liability standpoint. I have asked the same question brought up by Mike Huber about whether our customers understand, and for this reason it might be a good idea to explain the distinction upfront.
About the NOTE...I think it's fine when it's called out in text, but it gets annoying to see NOTE over and over again under a list of Precautions/Guidelines at the front of a manual. I've been trying to bullet "NOTE" items when they occur in a list.