It is the value of output from an enterprise less the variable costs of producing that output
For breeding livestock it is the value of milk plus progeny (calves or lambs) plus any subsidy, less the cost of replacing the breeding animal. With the breeding livestock enterprises (dairy cows, suckler cows and sheep) , the cost of replacing the breeding animal is the annualised cost as the animals normally last more than one year.
For non breeding livestock (eg finishing beef animals) it is the value of the finished animal plus any subsidy less the cost of replacing the animal (eg purchasing a store or a calf).
Costs that are sensitive to change in the size of the enterprise, in terms of crop area or livestock numbers, or sensitive to a change in the yield of the enterprise, eg milk yield per cow or crop yield per hectare.