Your Joomla website has a number of User Plugins working with the User Component.
Not all User plugins are used directly by your site visitors or Registered Users - but are User plugins in the sense of website User Management for example they are used by Joomla in the functionality of your registered user's Profile.
The 'User - Profile' Plugin gives the functionality of your Registered Users to complete their User Profile.
In Joomla, Plugins are managed via the administrator area.
Extensions - Plugin Manager - Edit User Group