It's called a Purchase to Pay system - whoever makes an order supplies a Purchase Order number from their internal system, which the supplier will reference on

It's called a Purchase to Pay system - whoever makes an order supplies a Purchase Order number from their internal system, which the supplier will reference on their invoice so the accounts team can look it up before paying it. In terms HN would understand, it's a stateful firewall for invoices that prevents paying orders that didn't originate from your company.