As with [[ https://www.mediawiki.org/wiki/Extension:CodeReview | CodeReview ]], to organize work.
See https://www.mediawiki.org/wiki/Gerrit/Tagging
The corresponding Gerrit feature is called hashtags (it's not well-documented but there is some information in [[https://gerrit.googlesource.com/gerrit/+/c03196df4d0eda7d85778646e940e62337227c20/ReleaseNotes/ReleaseNotes-2.11.txt#51|the release notes]] and [[https://gerrit-review.googlesource.com/Documentation/access-control.html#category_edit_hashtags|access control]] and [[https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#get-hashtags|API]] docs); it requires switching from ReviewDb to [[https://gerrit-review.googlesource.com/Documentation/note-db.html|NoteDb]] (basically, using metadata embedded in the git repository instead of SQL).