A department contact can be used if you need to send your invoices via email to a department and not to a real person. A name etc. is not required using department contact. To create a department contact, just mark the contact as ‘Department Contact’ and insert the data.
If you create afterwards an invoice or estimate the contact will be visible to choose. You can also make a department contact as primary contact.
For a contact as ‘Private Person’, the department contact is not available.