Task lists: cost element requiered

Hi experts!!!

when creating a task list (transaction IA05) and going to operation view, if I use control key SM03, the system requieres to enter "cost element".

The thing is that if I create a work order and I enter an external operation by using controk key SM03, the system does not requiere to enter "cost element". And if you go to the customizing of task lists: plant maintenance and customer service --> maintenance plans, work centers, task lists and PRTs -->task lists -->control data -->define field selection the field cost element is not set as requiered.

Best regards