Folio level data (also known as, transaction data) is the payment, reservation and guest data that is based on the payment at check-out. Many hotel technology systems use reservation data (on the books data) to manage occupancy, rates and availability and while this is good information for future availability it doesn’t represent how the hotel actually ended the day and the actual revenue generated. There are many changes that can happen between a reservation made and the actual check-out, last minute price changes etc which aren’t reflected on reservation data.
A guest reserves 3 nights but finally only stays 2 nights. Or a guest reserves at best available rates but requests an additional discount on check-out.
As an example in other industries it would be measuring a company’s health based on orders received rather than invoices issued.