"Whole Known Network" is what was previously called the "public timeline", and is different from the local timeline. It's a unique feature that displays public posts from all subscribed users, from different instances -- even if you don't follow them. The WKN timeline is different from your home timeline, which only displays posts and boosts from people that you follow. The WKN timeline will only display in the last column if you click on it. It updates in real time as other users post new toots. If you keep the scrollbar at the top of the column, new posts will push older posts down. If you have scrolled down, your spot in the timeline will be preserved, even as new posts are added. To view the newer posts, simply scroll back up.
People you don't follow, or perhaps don't even know, are likely finding your posts via the WKN timeline, or the local timeline. By default, all public posts from a user are displayed on these timelines. However, for users that do not wish to have their posts displayed there, there are privacy options available to you in order to control how you would like to share your content on Mastodon. Private posts, posts marked to not display in the public timelines, and boosted posts will not display in these timelines.
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