Overhead Key
Specifies which overhead is applied to a reference object (such as a material), which forms a link between overhead conditions and the following:
A material master record
A sales order item
You can define special overhead conditions for certain reference objects.
Overhead Key for Materials
In order to link materials to certain overhead conditions, you need to:
Enter an overhead group in the costing view of the material master record
Enter an overhead key in the costing sheet that is linked to this overhead group in Customizing for Product Cost Controlling
Based on the overhead key, the overhead is assigned to a particular material through the overhead group in the costing view of the material master.
The overhead group and overhead key are included in the following:
Material costing in Product Cost Planning
In Cost Object Controlling:
In a preliminary cost estimate for the product cost collector or for the manufacturing order
In period-end closing for the product cost collector or for the manufacturing order
Overhead Key for Sales Order Items
To link sales order items to overhead conditions, enter an overhead key for the sales order item. To do this, go into the sales order and choose 
Extras
Account assignment
.
The overhead key is included in Product Cost by Sales Order:
When the planned costs of sales orders are calculated
When overhead is calculated at period-end closing