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.
SYSAdmin can configure in Company Settings whether to show User Card information by default.
Users can turn it off by clicking the gear icon at the top right of the User Card or adjust the display settings through the Preferences in Personal Settings.