Once the batch order is finished and the true-up calculation is executed, the material consumption details (estimate vs. actual) and price changes are visible if the user clicks the True-up details button in the On-account form (Project management and accounting > Projects > All projects > Manage > Bill > On-account transactions). Non-changeable items usage is identified by checking if the inventory status of the consumed material is marked as Chargeable.
The Creation control parameter (Project management and accounting > Setup > Project management and accounting parameters > Invoice) is controlling whether a warning or error message should be raised in case there are already upfront invoicing related milestones in the billing rule.