What is a primary department?
If a primary department is selected for a coworker, this department will automatically be selected for documents added by this coworker.
You don't have to choose a primary department for a coworker. If coworker who hasn't been assigned a primary department adds a document, you can easily manually add a department in the Department field on the document:
How do I assign a primary department to a co-worker?
Go to Settings > Team, and click the desired co-worker. You can now select a primary for that coworker:
You can choose from departments that were imported from your company's accounting platform via an integration or that were have created manually in Settings > List > Departments (if no departments were created, the ability to select a primary department will not be visible).
You can also use the company's workflow to automatically select department for new documents. Read more about setting up workflow here.