From Chapter 15, Starting a Kanban Change Initiative:

Kanban strikes a different type of bargain. Kanban does not seek to make a promise and commit based on something that is uncertain. A typical Kanban implementation involves agreement that there will be a regular delivery of high-quality working software -- perhaps every two weeks. The external stakeholders are offered complete transparency into the workings of the process and, if they want, daily visibility of progress. Equally, they are offered frequent opportunities to select the most important new items for development. The frequency of this selection process is likely to be more frequent than the delivery rate -- typically, once per week, though some teams have acheived on-demand selection or very frequent rates such as daily or twice a week.

The team offers to do its best work and deliver the largest quantity of working software possible; and to make ongoing efforts to increase the quantity, frequency, and lead time to delivery. In addition to offering the business incredible flexibility to select items for processing in very small quantities, the team may also offer the business additional flexibility on priority and importance by offering several classes of service for work. This concept is explained in chapter 11.