The outstanding invoice reminders enables you to set up your own customised emails, SMS, letters and tasks to trigger automatically whenever an invoice becomes (X) number of hours, days or months either before the due date, after the invoice is due and triggering from the invoice date itself.
Once they're set up, they will automatically chase outstanding invoices for you.
This can be set up in the Web Portal > Settings cog > Smart-Assist > Invoices - Accounts Assistant > Scroll down to the Outstanding Invoice Reminders > set up your Invoice reminders > Save at the bottom of the page.