Original Estimate Mode
This is the default effort mode in which the Original estimate is evenly distributed over the task's duration, taking into account Resource's non working days. The non working days can result from Workload plans, Holiday Plans or the Working schedule.
Original estimate is a 'Time tracking' field so to add it to your issue screen add the 'Time tracking'.
The video below show an example of the calculation using a task with 1w (40h) Original estimate. Please note that in this example date picker type of fields are used to determine the start and end position of the task. Also, using drag and drop a different resource - Calvin - is assigned to the task. Calvin is available for only 3d in this particular week hence the tasks duration is adjusted. Another thing worth mentioning is that Angela will not appear on the Resource list as she is not assigned to any tasks in the specified period.
If the Task does not have Original Estimate then the warning shows up (a red line that contours the Task) - considering 'Show warnings' feature is enabled