To update the Department for the Item lines in the Vendor Bill, the following columns should be present in your CSV file:
Internal ID - for Vendor Bills
Amount - amount pertaining to Items (not the total Transaction Amount)
Department/Class - may be Internal ID or Name
Item - may be Internal ID or Name
Location - if mandatory
Order Line - order line for Items
The following should be the mapping:
Internal ID = Vendor Bill : Internal ID
Amount = Vendor Bill Items : Amount
Department/Class = Vendor Bill Items : Department / Vendor Bill Items : Class
Item = Vendor Bill Items : Item
Location = Vendor Bill Items : Location
Order Line = Vendor Bill Items : Order Line
*If you used Internal ID, click on the Pencil Icon > choose Reference Type = Internal ID