Question: (<https://webapps.stackexchange.com/review/suggested-edits/111499> shows action by both.) [Edit: At first glance they both looked like special users. Not so.]

Answer: 
Community♦ is [a special user](https://meta.stackexchange.com/questions/19738/who-is-the-community-user). Community is credited for reviews when they are:

* Done by deleted users
* Improved, in which case Community is shown as having approved the edit
* Rejected and edited, in which case Community is shown as having rejected the edit

[user0](https://webapps.stackexchange.com/users/186471/user0) is a normal user who just happens to have "0" in the username. user0 approved the edit before it was improved by a different user.
A key difference is that their real user numbers, respectively, are -1 for Community♦ and 186471 for user0 (on webapps, and thus *not* 0), as can be seen in the URL (classically displayed in a browser's status bar when mousing over the usernames) that clicking on the name opens. 

[Community♦](https://stackexchange.com/users/-1) (/community is tacked on the end of the canonical URL) is [a special user](https://meta.stackexchange.com/questions/19738/who-is-the-community-user).

But [user0](https://webapps.stackexchange.com/users/186471) (/user0 is tacked on the end of the canonical URL) is a normal user who just happens to have "0" in the username. user0 approved the edit before it was improved by a different user. 

[I found no true user 0 - no user with ID 0](https://stackexchange.com/users/0/anything) on any SE sites.  

[There is also no user with ID 1](https://webapps.stackexchange.com/users/1) on some SE sites, but
[there IS a user with ID 1](https://stackexchange.com/users/1) on other SE sites.