Imagine you've been to the first part of a two-part lecture. Now you can carry on recording from where you left off at the end of the first lecture by going back to the same page, clicking the page, and then starting a new recording (the date and time will be added wherever you have clicked the page). The page will show two pieces of "recording started" text inserted by OneNote, but the Audio gauge will appear to have one long recording, though in fact two separate files will be created by OneNote. You can start and stop as many times as you like on a single page (or page group).
The timeline on the Audio gauge represents the times of all the files strung together. Playback continues from file to file, switching between playing video and audio as needed.
You can also click the recording date and time text that OneNote inserted to jump to that part of the recording. This is a great navigational aid if you have numerous recordings strung together.