You can tag text frames and graphics automatically or manually. After you tag page items, you can use the Structure pane to change the order of your page by dragging elements to a new location within the hierarchy. If you change the order of the elements in the Structure pane, these changes are passed on to the Adobe PDF file. The order of the elements becomes useful when the PDF file is saved from Acrobat as an HTML or XML file.
When you choose the Add Untagged Items command, InDesign adds tags to the Tags panel, and applies the Story and Figure tags to certain untagged page items. The Story tag is applied to any untagged text frames, and the Figure tag is applied to any untagged graphics. You can then manually apply other tags to sections of text. However, automatically tagging page items does not guarantee that the items will be structured accordingly in the exported PDF file.