When browsing Info files, all local relevant information is displayed in the info buffer and a topical high-level view is provided in speedbar. All top-level info nodes are shown in the speedbar frame, and can be jumped to by clicking the name.
You can open these nodes with the ‘[+]’ button to see what sub-topics are available. Since these sub-topics are not examined until you click the ‘[+]’ button, sometimes a ‘[?]’ will appear when you click on a ‘[+]’, indicating that there are no sub-topics.