AX 2012 Purchase Variable Overhead

In the standard Contoso demo data the Costing sheet setup includes a COP - Cost of purchase node which defines a 2.25% Purchase variable overhead. I have just created a new item and posted a product receipt and the purchase overhead is applied, but how? The cost group on my new item is blank so where is the configuration to define that the cost node is a purchase overhead and is applied to items with a blank cost group?

Thanks

Tim