To allocate overhead rates to a WBS element, you must assign a costing sheet to the WBS element. A costing sheet is a tool that allows you to define and calculate overhead costs based on various criteria, such as plant, company code, material origin, or order type. You can assign a costing sheet to a WBS element in the Control Data tab of the WBS element master data. Reference: https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/4dd8cb7b1c484b4b93af84d00f60fdb8/df72bb53707db44ce10000000a174cb4.html

Billing plans can be created for WBS elements and sales order items. Billing plans are used to define billing dates and billing amounts for periodic or milestone billing. WBS elements can be assigned to billing plans if they are assigned to a sales document item or if they have a billing element indicator set. Sales order items can be assigned to billing plans if they have a relevant item category and billing relevance. Reference: https://help.sap.com/doc/0f069b2e08b748f1b480d4c8b46c3c49/100/en-US/6a7a9f0a6e421014e10000000a174cb4.html