Is Department a multiple-value enabled choice column? If so, then the settings should be:
Dept_ComboBox.Items = Choices(SharePointList1.Department) Books_ComboBox.Items = Distinct(Filter(SharePointList1, First(Dept_ComboBox.SelectedItems) in Department && Last(FirstN(Dept_ComboBox.SelectedItems,2)) in Department&& Last(FirstN(Dept_ComboBox.SelectedItems,3)) in Department && ...... ) ,Books)
How "......" shows depends on how many options you have for this choice column.
Community Support Team _ Mona Li If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.