Using the department property, you can mark up which departments use or have ownership of a specific vendor. For example you can register that marketing is the main user of the vendor Google and finance is the main user/owner of the vendor Pleo.
A subsidiary property could help you keep track of vendors only used in certain branches of your organisation. Lets say your company has branches in Germany, Denmark and France. Maybe the French branch uses vendors that are exclusive to their branch. This will help you keep track of those vendors.
This makes it easy for you to keep track of where the different vendors belong and how they are used in your organisation.