Summary

There are two reasons why the system generates an Invoice, and both are in the Work Order:

Key Points

The Billing Frequency is set to create a new invoice on a specific date; either For Every Work Order; if it is a series, Next Invoice Date

If it is Next Invoice Date, the billing and service are 'split' and there may not be a Work Order available.
The Work Order status is updated to Complete either online or in the mobile app

Both of the above triggers the system to generate an invoice.