Gmail's code includes some new smart labels for travel, receipts and events. I've tried to search for label:^smartlabel_event, label:^smartlabel_receipt, label:^smartlabel_travel and they don't seem to work.
Google already categorizes your Gmail messages to enable the features added to Google Now a few months ago. Google needs to identify confirmation messages for restaurant reservations, hotel bookings, flights, events you purchased a ticket for, online orders and it's obvious that Gmail could create smart labels for each category.
