When the cursor hovers over the Select User field, user avatars in Comments, user avatars in Tasks, and @Mentioned users, the basic information of the respective users will be displayed. It includes the user's name, email, phone/extension/mobile, online status, photo, job title, and department.
Note: External Users cannot see User Cards, so there is no need to worry about data leakage.
This information comes from the User Page.
Users can configure whether to display their User Card information through the Preferences settings in the Personal Settings.