What does the "Caution" box mean on the sectional near Half Moon Bay, California? What does this Caution box on the sectional chart near Half Moon Bay, California mean? Is there an official reference for the meaning of this box?
This one is a bit confusing and interestingly there is nothing in the FAA's sectional guide on "Caution" boxes although they do appear on other charts as well. 

This one in particular is a warning that you will need monitor the radio for the transition routes below 2000 ft. If you can zoom in or get TAC level map resolution you will see a different warning 



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If you note a bit south of half moon bay the chart makes note of the fact you should be using the TAC in that area: 



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