summary

A  summary  is short text that conveys a post's main point(s), a small representative section, a post excerpt, or a plain text equivalent for kinds of posts like likes and RSVPs that use other properties to convey their meaning.

Why
To explicitly indicate what part of the content of a post, or other text equivalent, should be used as a shorter description of the post in readers, search results, other link-preview use-cases for displaying a post from elsewhere, providing an excerpt for POSSE purposes, and a plain-text fallback for special kinds of posts that the consuming code might not know to do anything special with.

How
Add the class name "p-summary" to the element (e.g. or etc ) tag that you want used as the plain text summary for your post.

Kartik Prabhu
on https://kartikprabhu.com/ uses "p-summary" on shorter descriptions when displaying articles in a list and also, in the full display of articles. The summary is entered into a Django database UI manually for the list but, is also present as a "p-summary" in the HTML for the complete article.

Kevin Marks
Manually adding summaries on http://kevinmarks.com Also, unmungwill create summary from feeds, and display summaries in storycards

Bridgy
Bridgy Publish will look for  in your original post before   or "In general, Bridgy prefers to use p-summary if available, then the full e-content, and finally p-name, in that order. The one exception is original tweets (not @-replies) on Twitter: for those, it prefers p-name before e-content."

UI for authoring summary
Why a summary UI: because you may not want the entirety of your original post going to silos like Facebook or Twitter, and prefer to send a short summary to them and have readers there click through to your original post to see it in its entirety.

Implied UI auto-summary
Ideally it would be possible to create an algorithm that determined a summary (longer than a tweet, but not the whole thing) in a predictable enough way that you could "just" write your note, knowing that the (???) part of it would automatically become the summary.

Some thoughts:
 * auto-excerpt:
 * first paragraph
 * first paragraph or list break after 140 characters

This could be highly personalized, that is, an algorithm that an author chooses (perhaps from a few choices) because they may be more familiar with their writing style and how effective any particular algorithm would be without having to alter their writing style.

More text UI
Why a new summary indicating UI: I wanted a better UI solution for authoring a p-summary than having to explicitly author  markup, or have a separate text field for entering a summary. - undefined

Thoughts (undefined)
 * The author could simply put the explicit text content "More..." or "More:" in its own paragraph as a way of indicating that everything in "content" before the "More" should be considered the summary.
 * The consuming code (your authoring tool) could remove the "More" text and automatically markup wrap the text before it with:

Summary for private posts
When publishing certain private posts, one can use a summary to publish some information about the private post, announcing it's existence, but not publish the whole content. Such a post can be identified as an iceberg post.