You will notify a particular colleague to read a comment in the discussion or to answer one question for example. You do not have to assign the whole task. It is one-time notification.
One-time user notification
In the editor, type the at-sign. It activates the list of users – with the authority to this project/task – and you can either use the keyboard or gradual typing to narrow down the number of people on the list.
What will appear to this user?
That user will then receive a notification of the mention. E-mail alert depends on its settings, so it does not always get it. However, notifications at the notification center will always appear in the app.