You want to create a maintenance order with an external order operation with service items. Which of the following state

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You want to create a maintenance order with an external order operation with service items.
Which of the following statements are correct? (Choose three.)

A. The assignment of a control key with the service indicator set is mandatory
B. The assignment of a service master record is mandatory
C. The assignment of service master records to an order operation is optional
D. The usage of a model service specification is mandatory
E. The actual value entry can only be recorded via the service entry sheet