User blog comment:MisterWoodhouse/The Unified Community Platform Editor and our first release date/@comment-26569706-20200309170806/@comment-24171918-20200309172036

I guess this is for inserting HTML comments (in source code, this would like this: ) inside the page. Meaning they are invisably normally but appear while editing. So you could e.g. instruct other editors what to keep in mind when changing something or what a line of code in source mode would do.

This is already possible to do but it doesn't show up in current visual editors.