Unlisted (aka Quiet Public) – Anyone can see it, but it won’t appear in the trending posts list or the Local or Federated timelines, and it won’t show up in search results. This can be useful for replying in threads, so that you’re not filling people’s timelines unnecessarily. Normally indicated by an open lock icon or a crescent moon.
https://fedi.tips/who-can-see-my-posts-in-mastodon-how-do-i-send-dms-in-mastodon/