A space contains 2 values that can be affected by a calculation modifier: the area and the perimeter. You can choose to have a calculation modifier apply to either or both of these calculations.
For example you might want to set up a modifier that reduces the calculated base area by 50 percent. (This is obligatory for several space types in various international architectural standards.)
Or you might want a modifier that deducts length from the perimeter of a room for openings and stairs. This would be helpful in calculating the amount of baseboard material needed for this room.
- Click
. 
- Expand Documentation Objects, and expand Calculation Modifier Styles.
- Select the modifier for which you want to set the calculation type.
- Click the Apply To tab.
- Select whether you want to apply the modifier to the (base) area or (base) perimeter of the space. You can also select both values here.
- Click OK.