The term service class is usually used in regard to Kanban. Basically, tasks in Kanban systems are assigned to one of several service classes; these are risk classes based on the criterion of costs that are caused by delay. In order to establish the risk class you start with the question "What does it cost the company if a task is not completed until 5, 10, 20 or 50 days later?" It is obvious that tasks with high risk costs (i.e.: delay costs) should be processed particularly quickly.
A classification of service classes comprises the following: "Accelerated", "Fixed delivery date", "Standard" and "Indeterminable costs". Tasks in the "Standard" service class should make up the bulk of the work; if this is not the case, processes, staffing and the like should be subjected to a fundamental review.