There's now two sources of notifications because they're different in purpose.
Mailbox is messages directly to you - replies and direct messages. Same as before.
The Notifications/Bell is a different type of notification feed. It has what is meant to be positive notifications in it (new followers, reactions, mentions). It'll also be a place for other types of useful notifications that we'll add: price alerts, news alerts, rewards, etc.
X splits theirs in a similar way.
Market Data 
Markets 
AI
