On Page downloads the entire page, including any "boilerplate text" - headers, sidebars, footers and other text that is not strictly part of your content.
In Content uses machine learning software that grabs only the core content (important text blocks) from a page, stripping any boilerplate text.
The algorithm used by In Content was designed to detect meaningful content - usually at least one paragraph, but usually multiple paragraphs.
Our recommendation is, in order to cover a topic comprehensively, write meaningful text describing the topic.
Partial content, such as a list of keywords or sentence fragments, is not nearly as helpful to users, and search engines don't value it as highly, either.
When in doubt on which mode to use, use On Page, as it casts the broadest net.