Billing Plans and Assembly Processing 
Use
You use billing plans to plan the dates on which payment requests or invoices affecting sales are sent to customers.
Features
There are two ways of creating billing plans, if you are working with assembly processing and the Project System:
Billing Plans.

You cannot create a billing plan in a standard work breakdown structure as in an executable work breakdown structure.
Milestones
Milestones have the advantage that billing is only triggered when a particular activity or WBS element is reached. This means that you can associate billing documents with certain phases in your project, for example signing the contract or completing design. If any phase of the project is delayed for any reason, the milestones are rescheduled accordingly. This prevents billing documents from being created too early.
When you create an assembly order using a standard network that contains billing milestones, the dates of the milestones are copied to the billing plan for the sales document. If you then create or delete a milestone in the executable network, the billing plan is automatically updated.

Although you can create billing milestones in a standard WBS, these are not copied to the billing plan in the sales document.
You cannot use dates that refer to milestones in another network.
Activities
For details on milestones in the Project System, refer to
Milestones.To use milestones in billing plans in sales documents, make sure you set the Sales document date indicator in the milestone. Otherwise the milestone is not copied to the billing plan.