fragment

A  fragment  is part of a URL, and typically refers to a fragment identifier at the end of a URL preceded by a # but may also refer to a media fragment which may use a fragment identifier or a query string.

Webmention and fragments
The development of marginalia has demonstrated that Webmention can be used to comment on specific parts of the content of a post.

This likely makes sense to document explicitly as a Webmention extension, i.e.:
 * Webmention Fragment support
 * How and when a webmention sender should use a fragment in the source and or target URLs
 * How should a webmention receiver handle a fragment in the source and or target URLs

Where a fragment could be:
 * plain old URL fragment to an element with that ID in the HTML
 * fragmention
 * media fragment