Hello @Quinex,
Please understand that we merely provided a tool that bridges the gap between the site and the Schema for easier application. We would highly recommend that you understand the Schema as a whole so that it would be very clear when using the Snippet extension plugin. This is why as you noticed our knowledge base article (https://theme.co/apex/forum/t/extension-snippet/94) only discusses more about how you can use the Snippet plugin and not what it is about.
@Jade has responded and gave a direct reference related to Schema.
@Paul.r, on the other hand, suggested a link on how you can implement the MainEntityofPage entry to your site.
We are helping the best that we can and pointing out the Schema.org site is the right place to better understand how Schema works and what are the necessary things in order for it to work. Our snippet plugin is just a tool that you can use in applying Schema. In your case, it is either you insert Creative Work or a URL of the page.
Thank you for your kind understanding.