What does the Ignore URL action do?

When viewing 404s (eg. /wp-admin/tools.php?page=redirection.php&sub=404s), there are several options:

• Delete
• Redirect
• Do nothing (Ignore)

What does the “Do nothing (Ignore)” action actually do?

I can imagine several options:

1. The server sends no response to the HTTP request.
2. Further redirects are ignored for this HTTP request, and it is passed on to WordPress to respond to (probably with a 404).
3. Requests to this URL will not be logged in the Redirection plugin, but will continue to receive 404 responses.