When the Service Orders are completed, they can be released for billing. At first, you create Billing Document Requests. After that, you create Invoices to charge the customers.
You use the Release for Billing SAP Fiori app to check which items need to be billed and release them for billing.
Afterwards you use the Create Billing Documents SAP Fiori app to create Invoices.
You check whether the Invoices contain the correct Items for billing and post the billing documents. The system automatically creates Journal Entries for Accounting.