expression, noun which may take the genitive case particle 'no'
now available; currently released; now showing
Used on notices, websites, advertisements, and listings for things currently open or released to the public, such as films, exhibitions, documents, videos, or sales offerings. It can modify a noun with の.
新作映画は全国で公開中です。
The new film is now showing nationwide.
調査結果はウェブサイトで公開中です。
The survey results are currently available on the website.
公開中の展示を見に行った。
I went to see the exhibition that is currently open.
非公開 means something is kept private or not released to the public, the opposite of 公開中, which means it is currently open to public access.
A transparent compound of 公開, meaning public opening or release, plus 中, meaning being in the middle of a state or currently ongoing.