superuser is a privileged user on a site (typically a silo, like Foursquare) that has some admin abilities, sometimes different based on what "level" of superuser the user has been granted or has achieved.
Consider implementing superusers on your site so you can delegate some amount of admin or moderation to others.
TBD. See existing examples below and add some brainstorming.
On 2016-11-18 sometime (mid-day EST) Foursquare reset all SU3 passwords for security reasons. No specifics were provided.