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
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: