This column appears if the setting Use operation overflow? has been configured under the heading Preparation that you will find under the Manufacturing tab in the Settings procedure. Then you can use overflow in the operation list when preparing a part.
If it is more appropriate to use a quantity in percentage form instead of a fixed quantity in figures, this is where you enter a percentage quantity for the overflow.
This can be used as an alternative to, or in combination with the Setup quantity on the operation row.