What is the meaning of the various post messages?

I’m confused about some of the messages that can appear on a post entry/edit screen in the admin area:

$messages['post'] = array(
         0 => '', // Unused. Messages start at index 1.
         1 => __( 'Post updated.' ) . $view_post_link_html,
         2 => __( 'Custom field updated.' ),
         3 => __( 'Custom field deleted.' ),
         4 => __( 'Post updated.' ),
        /* translators: %s: date and time of the revision */
         5 => isset($_GET['revision']) ? sprintf( __( 'Post restored to revision from %s.' ), wp_post_revision_title( (int) $_GET['revision'], false ) ) : false,
         6 => __( 'Post published.' ) . $view_post_link_html,
         7 => __( 'Post saved.' ),
         8 => __( 'Post submitted.' ) . $preview_post_link_html,
         9 => sprintf( __( 'Post scheduled for: %s.' ), '<strong>' . $scheduled_date . '</strong>' ) . $scheduled_post_link_html,
        10 => __( 'Post draft updated.' ) . $preview_post_link_html,
);

For instance, what is the difference between indices 1 and 4? When is one called, versus the other? Also, what is the difference between indices 7, 8, and 10? They sound like they would appear if you’re saving a draft of a post that has not yet been published (which would trigger #6.)

I haven’t been able to find any kind of guide for which message occurs at what time.

Thanks for shedding some light on the subject!

Good question! Let’s shed some light at this.

First of all, it’s worth noting that these messages can be overridden via the post_updated_messages filter. Thus, they could be slightly different depending on which plugins you use.

There are different functions responsible for appending &message=<number> to the URL (which determines the message being shown). You might have never seen this query parameter because WordPress removes it on page load (see wp_removable_query_args()). This way, you’ll never see the message again when you simply reload the page.

1. Used when updating an already published post.
   Function: redirect_post()
2. Used when you add a new meta value to a post and click on “Add Custom Field”.
   Appears to be only used when JS is disabled.
   Function: redirect_post()
3. Used when you delete a single meta value by clicking “Delete”.
   Appears to be only used when JS is disabled.
   Function: redirect_post()
4. This appears to be the fallback message when the origin of the request is unclear.
   The user will be redirected to the edit post screen.
   Function: redirect_post()
5. Used when restoring a specific post revision.
   File: wp-admin/revision.php
6. Default message when you publish a new post. Includes the the link to view it.
   Function: redirect_post()
7. This message doesn’t appear to be used right now.
   I might be wrong, tough. I’ll update this if it turns out it is used.
8. Default message when you save a post as “Pending Review”.
   Usually the case when you don’t have the capability to publish posts on your own.
   Function: redirect_post()

9. Default message when you schedule a post for a future date.
   Function: redirect_post()

10. Default message when you you edit a draft post. Includes the link to preview it.
    Function: redirect_post()