Is the flight from Boston to Bermuda domestic or international?

Answer 1

Both. US Customs, Immigration and Agriculture pre-clear passengers in Bermuda before boarding flights to the US, so they are treated like domestic passengers when arriving in the US. They must go through normal border controls before boarding the flight, however.

Freight shipped on the same flight will, as far as I am aware, be treated as international freight, having not been pre-cleared in Bermuda.

Passengers flying to Bermuda from the US must clear HM Customs and Bermuda Immigration on arrival.