There are a few potential reasons that a Report will list 'Unknown Department' against stock items.
Deleted Stock Item
If a Stock Item sold in the Report Date Range has since been deleted, the sale will appear under 'Unknown Department'.
Stock Item with Invalid Department
If the Stock Item is not assigned to a Department, it will appear on the Report under 'Unknown Department'. It is not normally possible to enter a Stock item and have no Department against it, so it's possible the Department was somehow removed/deleted at some point to cause this.
Customer Statement Account Keeping Fees
Any Account Keeping Fees will be listed as 'Surcharge' under 'Unknown Department'. These are fees charged to the customer, for running the account statement. They don't actually have a department.
Delivery Fees
Delivery Fees (Newsagency Only) will be listed as 'Delivery' under 'Unknown Department'. These are fees charged to the customer, for delivery of magazines or papers. They don't actually have a department.