What is the difference between an @ mention and an assignment?
With the mention, you draw someone's attention to a topic in a group. You let them know that you would like them to react or are waiting for a response from them. By mentioning, you generate for the mentioned person a notification or mobile push or email, so this action should be used sparingly. You can also mention @Group (all members) ("@"+"group or all" to notify everyone).
With the assignment, you specify to your correspondent that you expect an action from him/her. The action is logically explicit in the post and can also be accompanied by a "check list" (to do list). The person withdraws from the assignments when they have completed the action, leaving a comment on the post if necessary, specifying what they have done. The assignment also allows you to access a filter in the action thread to find all the posts you have something to do about.