8. There are 4 post types on mastodon.
PUBLIC is visible everyone.
UNLISTED is visible everyone too but is not listed on local and global timelines (replies to toots UNLISTED by default)
FOLLOWERS-ONLY posts are locked to only people who follows you.
DIRECT goes to only people mentioned on the toot. Note that admins can see the direct messages, think of it less as secret and more as not spamming everyone on your follower list by a long conversation.