Explain shortly the expressions below:

Double entry bookkeeping ( double entry accounting ) in the balance of payments; along with your definition, demonstrate it with the potential US balance of payments categories of NX (net export), BPI ( balance on primary income) , BSI ( balanced on secondary income) , KA ( capital account) ,NRFAnc ( the net credit non reserve financial account ) , and OSB ( the negative of the official settlements balance ) , and the scenarios that a US firm exports candy bars to ltaly receiving euro in return.