You may know that you can add comments within Google format files (Docs, Sheets, Slides, etc.), but you can also assign a specific task within the file to a colleague. You can also re-assign an action item to another user.
To do this, open your file and select Insert, then Comment. If you wish to assign an action to someone, type “+name” and select the correct person. This will generate an email to the person letting them know that they have been assigned an action.