How to Show or Hide Other Users’ Posts in WordPress
By default, WordPress users in the admin area can see all the Posts on the site, regardless of whether they are the author.
This is not a problem for many sites. After all, most Posts on most sites are publicly available – there's no need to hide them.
However, in some situations, site owners don't want authors to see the Posts that other users are working on. For example, one PublishPress customer wanted to create a private inbox for WordPress users.