There are five Assistant sections: Subscriptions, Travel, Packages, Bills & Receipts, and Entertainment. They scan through incoming emails to find any and all material that belong in their folders to help keep you organized.
It's important to note that the Assistant only scans messages which reach the inbox. If you have server-side rules which are moving messages to folders before they can reach the inbox, they will be missed by the Assistant.
If a message from the inbox has been missed by the Assistant, please forward the message to email@example.com, and include a note about how it should've been categorized.