A queue is the number of hours that a work order sits in front of a work center before it starts. It also includes the move time required to physically move partially completed items from the previous operation to the next work center1. A queue is used to provide a buffer between the time that a component arrives at a machine or work center and when the operation actually starts2. A queue can help to neutralize delays in previous operations and to reduce the waiting time and flow time of the work orders3.

An order that moves into a work center on schedule following completion of a previous operation will move into the queue in priority sequence. This means that the order will be placed in the queue according to its priority level, which is determined by the priority control technique used for the production system. A priority control technique is a method of determining the sequence and timing of production orders in a manufacturing system4. Some examples of priority control techniques are first-come, first-served (FCFS), shortest processing time (SPT), earliest due date (EDD), and critical ratio (CR). Depending on the priority control technique, an order may move to the beginning, end, or middle of the queue.

An order will not necessarily move to the end or the beginning of the queue, as these positions depend on the priority level of the order and the other orders in the queue. An order will not move into the queue when there is capacity available, as this implies that there is no queue at all. Capacity is the amount of time or resources available for production at a work center5. Capacity can be affected by factors such as machine availability, labor skills, setup time, and maintenance. Capacity planning is a function of determining and adjusting the optimal level of resources needed to meet the demand5.