Tagging specific posts in a thread so they can be easily navigated to.

This is useful for long threads discussing a topic over a lengthy period of time and many pages. A threadmark allows a user to quickly jump to a specific post which is related to a certain event, date, or whatever is relevant to the thread.

Optionally this allows building an 'index' of posts and allows easy next/previous navigation between marked posts. A threadmark index can then be attached to a threadmark list for additional information and statuses.