How Siebel Assignment Manager Creates Activities for Employees During Availability-Based Assignment
If an assignment rule has the Check Employee Calendar flag checked, then availability for each employee is evaluated before assignment. If the Calendar Create Activity property is also set for the assignment object, then make sure that an activity exists in the employee's calendar for that duration to block the schedule. The following figure shows a high-level flowchart of this process, which includes the following steps:
If the calendar’s Check Employee check box is selected for the Assignment Rule, then Assignment Manager . . .
Receives a candidate list comprising all employees who are available for the Period Start Time/Early Start Time and the Duration.
Evaluates each candidate against the assignment rule criteria.
Asssigns the object to the highest scoring employee from the list.
If the calendar’s Create Activity property is defined for the assignment object, then . .
An activity is created for the employee’s calendar to block that employee from further scheduling for the Duration Period.

Assignment Manager evaluates whether the Check Employee Calendar flag is checked (calendar rules) or not (noncalendar rules) and processes the rules differently based on this indication. If Assignment Manager is assigning activities, and if only one employee passes and that employee is from a calendar rule, then the original activity is reused and the employee is added to the owners list for that activity. This action blocks the employee's schedule. If more than one employee passes, then separate activities are created for each employee. In this case, the parent activity of all these additional activities is set to the original activity to make sure that all are related to each other.
If Assignment Manager is not assigning activities, then separate activities are created for each employee who passes and blocks their schedules.