This column appears if the setting Use material overflow? has been configured under the heading Preparations that you will find under the Manufacturing tab in the Settings procedure. This will allow you to use overflow in the BOM list when preparing a part.
You can enter a percentage value for material overflow, if it should be reported separately for the material quantity that is used for the part/parts. For example, when blank cutting or punching a part that weighs 2 kg from a 2,5 kg piece of sheet metal, you should enter 2 (kg) for that part in the Quantity column. For the metal sheet material overflow of 0,5 kg, enter 20 (%) under Extra(%). This percentage value will then be applied when placing orders in reference to the material reservation made for that part.
Please note that this only applies to overflow that will affect the material reservations. It does not apply to overflow from for example sheet metal that has been punched or cut out.