The need for modifications typically arises as a result of new technology, safety demands, quality demands, etc., that the manufacturer of an equipment has specified or developed (as a Service Bulletin), or because of repeated errors or damages. It may also be a result of required changes issued by the authorities (as an Airworthiness Directive). A modification leads to an occasional maintenance event that will be executed, within certain time limits, on all or selected serials. The modification process can also be used to control other types of one-time events that will be executed on all or some serials.