When you are writing a message in an activity, you can type the @ key in any chat window, and select from the list those you want to notify.
@name: Notifies specific people in a discussion.
@all: Notifies all participants in that discussion.
@online: Notifies all participants who are currently online.
Selecting from this list will produce an @mention directed at certain participants. Usually an @mention would trigger a push notification to desktops and mobile devices, but users do have the ability to customize their notification preference for each activity.
Internal Activities: All users default to @mentions required, which means if you want to get someone's attention, you would need to type @name or @all.
Private & Integrated Activities: All users default to @mentions for all comments, but it is not uncommon for people adjust their settings to @mentions required, so use @mentions if you want to get someones attention.
Direct Activities: All users have notifications set to on and this setting cannot be changed, so there is no need to use @mentions.