User blog comment:BertH/New on-site notifications/@comment-27702369-20170328143602/@comment-1824272-20170328180222

Not to mention, the notification system applies for the entirety of Fandom. Even if one community doesn't have Discussions, another might, and your one user account will be used for all communities you interact with. The Forum and Message Wall notifications are cross-wiki, and so is this new system they added.