The level of service is the centerpiece of the service level agreement that must be defined in service level management. It is a contract between the client and the service provider, which facilitates the control of recurring services for the client by setting the quality standards to be met. To define the optimal level of service, a call centre must identify the point at which, in a pre-defined setting, staff costs are as low as possible, but customer satisfaction is as high as possible. The following example shows the direct impact of a change in the level of service on staffing requirements (results are calculated using the Erlang C formula): mathematically, the level of service therefore describes the amount of orders and orders of customers that can be executed immediately from the warehouse.  For call centres, the level of service is the most important size to measure their accessibility. It expresses the percentage of calls received by all call officers in a given period of time. Thus, a service level of 90/10 means that on average 90% of all callers have to wait less than 10 seconds before being connected to an employee. As a standard level of service, the specialized literature indicates a value of 80/20 (see ABC analysis) that satisfies most customers. This standard level of service is only a reference value – it can vary depending on the priorities of the call center and customer expectations. From a linguistic point of mind, this is a fictitious anglicism, because American specialized literature means by level of service availability to delivery.  Both are however strongly distinguished in the German-speaking region. The level of service is derived from forecasting demand for sales planning and logistics inventory management.  The more accurate the forecasts and the more efficient the inventory management works, the more advantageous the level of service becomes and vice versa.
The level of service is mainly used for the purchase for standardization and therefore comparability for the most part when purchasing services.