The topics appear under the top line of the channels being tracked and essentially serve as a filter for the content you subscribe to.
The default is "All". This lists all videos from channels you have subscribed to, from the newest to the oldest. Then there is "Today", which shows all videos that have been published in the last 24 hours.
Other topics include "Continue Watching" (videos you’ve seen but not yet finished), "Unwatched" videos, "Live" videos (including live streams and YouTube premieres), and "Posts", that are shown Community contributions only.
According to Google, tests have shown that these topics encourage more subscribers to come to their subscription feed daily to check for new updates. However, they do not have to be used, and user feeds continue to appear in reverse chronological order if the topic buttons are not touched.
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