I believe this is a support type column. It really doesn't appear anywhere by default, but it is filtered out while using the same type of view.
The naming convention they have chosen is not very intuitive. Basically, when you assign a task to a user account, this field is blank. But if you assign a task to the group group, that is, the security group of the active directory, this field will be automatically populated by SharePoint.
SharePoint " ", , , " , , ".