Generally, it is the physically most remote area from the riser. HOWEVER, depending on height of sprinkler heads, it could be right next to the riser. An example would be a skylight or cupola that is protected. Or, it could be an area with higher demand, such as a higher hazard classification. You would have to calculate the demand/pressure for all such areas to determine which has the highest pressure requirements. As Oremus pointed out, go to the Code.