How companies work
In traditional accounting software, we differentiate customers and vendors as business relations for invoices, but many companies have many more relationships which we would like to register in our system.
Also, a company or person can have multiple relationships with our company. The best example is my relationship with Microsoft. Like everyone else, I use the software so I am a customer, both in my business and personal life. On the other hand, Microsoft hires me to teach workshops and do presentations, which makes me a vendor. As an MVP, I have a totally different relationship with them. They give me an award and invite me to special events and allow me to access the company store. They also ask for my advice on future versions, so to them I am their consultant.
So one person or company can have different roles in RM. Microsoft Dynamics NAV is able to handle all that while maintaining a single point of data entry and maintenance.