Main business data entities
To find out what the main business entities are, prepare and conduct a workshop with Igor, the information technology (IT) architect, and Melanie, the subject-matter expert (SME). They will probably have different perspectives on the data, and you will need to moderate accordingly to achieve the set goals.
Melanie knows all the user interfaces (UIs) in detail and can tell stories about the weird data her team sometimes needs to cope with. In her day-to-day business, she manages the validation process with suppliers and knows exactly what to do. She will need some support from you to get structure into the data she uses.
Igor knows all the involved database tables by heart. That structure seems to be way more complicated than you would expect from Melanie's description. To develop a simplified data model, Igor will need some help.
Note
It is your job to help them both to take a step toward each other and develop an understanding of each...