The Exterior Overcut is where the cut continues into the corner before retracting the blade from the matboard on the exterior cuts, such as when cutting a rectangle. 

Exterior Cuts cut into the matboard, not the fallout.

Make sure to select the desired Cartridge Tab and then the outside bevel type you want to edit from the Current Bevel Type dropdown control before making adjustments. Select Front of Mat if it is being cut from the front of the mat.

The Exterior Overcut is displayed on the Configuration Map as a red arrow.

Check against the Configuration Map of the Bevel Type you are cutting to determine specifically which cuts are the Exterior Overcuts.

Exterior Cuts, such as rectangles, cut into the matboard, not the fallout. 

If the exterior overcut retracts the blade too soon, it makes the exterior overcut too short.

Increase the Exterior Overcut.

If the exterior overcut retracts the blade too late, it makes the exterior overcut too long.

Decrease the Exterior Overcut.

For Overcuts where the blade comes out of the fallout instead of the matboard, see Interior Overcut.