Custom CNAME option allows you to use your own sub.domain.com address for files instead of default media.publit.io URL.
It can be set up as a simple DNS CNAME record or even as your own CDN pointing to media.publit.io and using it as origin.
The article on how to setup CNAME is available here https://publit.io/community/blog/custom-cname-cdn-for-media-files-urls-at-publitio