When adding a resource from the Library catalog, or when uploading your own PDF document to a reading list, you can choose which section you want to add the item to by clicking on the "Section" menu at the bottom of the right-hand sidebar. Choose the correct section and click "add" or "add & edit":
You can also move resources from other parts of your reading list into a section (or out of a section, if need be). Hover over the item you'd like to move and click and hold on the blue bar/panel that appears. Then drag the item to its new location:
Watch a video from Leganto demonstrating this procedure: