You should consider some other important properties when working with Activities:
Regardless of whether you set the duration and due dates (the fields you use to set the date when the Activity is or should be performed), Appointment and Service Activities are the only Activities that show up on the Microsoft CRM Calendar.
For more information about the Microsoft CRM Calendar, see the “Calendar” section later in this chapter.
After you close an Activity, you cannot edit or reopen it (with the exception of Campaign Responses). ...