If your practice wishes to process patient invoices for private treatment as a batch, so that you can print and send out the invoices on one day per month (for example), the process must be started at the time the treatment plan is marked as complete.
Check the Ready for Batch Invoicing on checkbox so that it is ticked, then click in the field to the right and select a date option prior to (or on) the date you intend to run the batch invoicing (see selection options indicated above). This will ensure this invoice is not included in an earlier run.