Here you can specify departments which the discount level should cover. This results in the discount being applied to the products from these departments. To add departments to the list, click the Add departments button and check the boxes next to the appropriate items in the displayed selection dialog. Department items can be removed from the list at any time using the corresponding checkboxes together with the Remove selected button. You can also remove all listed items at once by clicking the icon and performing the Remove all action.
More resources concerning departments can be found in E-commerce Guide -> Managing your store -> Departments.
Further information about discount levels can be found in E-commerce Guide -> Managing your store -> Discounts.