Reservation systems have been observed in Kanban implementations since 2008. They are used immediately upstream of a delivery kanban system, to indicate the desired start date of the request for work (a work item).
We don´t perform a specific activity to prioritize work for kanban systems. Instead we practice dynamic prioritization, selecting items of work on-the-spot, based on risk, or other desirable outcomes such as fairness, good customer service, or value.
Understanding what stands behind Classes of Service, even in their simplest form, gives people the Archimedes’ “eureka!” moment and opens new options for further discussion about improvements.