Invoices come from four places: orders, recurring services falling due, an administrator, and the API. Each carries the client, currency, tax, lines, balance, status and payment method.
Recurring services
A service has a billing cycle and a next due date. The daily automation raises its invoice ahead of that date (the number of days is a billing setting), sends it, and sends reminders before and after the due date. Late fees and suspension of an overdue service follow the rules you set under Automation Settings.
No scheduler, no recurring invoices
The daily automation runs from the cron job. If it is not running, nothing is invoiced on schedule. See Set up the scheduler.
Orders
An order placed in the store raises its first invoice straight away, including any setup fee. Its services wait as pending until the order is accepted under Orders.
By hand and through the API
Billing › Invoices › Create Invoice raises a one-off invoice for any client. Billable items, one-off or recurring charges added to a client's account, are invoiced on the date set for each. The CreateInvoice API action does the same for another system.
Full guide
The complete reference is Billing and invoices in the WBAMS documentation.