Atto has been approved for core for Moodle 2.7 although it has been available as a plugin since Moodle 2.5 was released.
Developer Damon Wiese says
“The Atto HTML editor is an alternative text editor for Moodle that aims to be fast, accessible, responsive and simple. (What does Atto mean? Very, very small. It’s the SI prefix for 10 to the power of -18). From Moodle 2.6 everyone can choose their preferred text editor in their profile, so you can have different text editors installed, and everyone can use the one they like best.“
I think people who use Moodle primarily on a mobile device will want to change their Profile preference to use the Atto editor now it’s available in Moodle core code from 2.7. However, I’m also fairly certain that many Teachers and Students using Moodle rarely visit their Profile page after they first create a Login, and many people will miss this new editor.