Post Edit Logging
Post edit logging is the simpler of the two features. It adds an indication to the post when it has been edited. We have intentionally kept this feature simple for the average user. They have no reason that they have to enter; if they're inclined, they can always include the reason in the message.
We do track the last person to edit the message, but this is not displayed. It could be easily added with an add-on. In most cases, the last edit will be by the owner and the average user likely doesn't care about who edited it. If a moderator needs to know, the history gives much more detailed information.
The edit log can be set to not be displayed if a post is edited in the first X minutes.
So what does this look like on a post?
Moderators also get a few extra options to control the public log of the edit. Note that these options do not affect the history. This only allows them to suppress the public notice.
Post Edit History
Post edit history actually keeps all previous versions of a message rather than just an indication that it was edited. This can be used for many things, including handling the "rage-delete" situation, where a user edits all of their content, potentially destroying the flow of many threads.
For developers, it's worth noting that the history system has mostly been developed to be content agnostic. Add-ons can make use of the system and automatically get access to the history and comparison interface.
Whenever a post is edited--even with a "silent" moderator edit--the history is logged. You may have noticed it in the screenshot above, but a history link will appear whenever a post has been edited.
When you click that link, you will have an opportunity to pick two versions to compare and see the actual changes made
Note that the UI I'm demonstrating below is still a work in progress and improvements will be made.
You can also view the previous version in its raw form with a button to the right (which I didn't include in the screenshot). Options to automatically revert to a previous version may be included.
History data can be set to only be maintained for a specific number of days.