We are still struggling with dates on the invoicing process, and I am trying to understand how the system is meant to work.

There are two date fields, described as follows in SuiteAnswers:

In the Next Bill On Or Before field, enter a date to filter the list by the billing date on the orders.

In the Date field, enter the billing date for the transactions. The current date is entered by default.

My first question is 'what is the billing date' - I can't find this field anywhere in the system?

It is my understanding (from the testing and issues we've seen so far) that a Sales Order will not be billed until it is Shipped (which is correct). However, I think the dates it uses is based on the Sales Order date rather than the date of Fulfilment.