Invoice unique #ID (for debtor)

Why does every invoice have a #number?

As you can see across the platform, every invoice number has its own ID number. It's basically the SupplierPlus invoice number. All the actions that are being done with the invoice (like approving and selling the invoice), are being saved in Action history. 

You might have many different suppliers who have an identical invoice number. SupplierPlus invoice number or as #number will help you to distinguish these invoices. You can also use #number in search feature.