There are tasks for which it is not possible to simply divide the efforts by the number of resource to calculate the duration.
By setting this schedule calculation type for each task, the duration is automatically calculated by the calculation method according to each type.

I think there are 4 pattern to calculate the schedule.

1. Effort based calculation. (Default)
Duration = Effort / Resource * Work hours a day
Efforts = 40 H
Resource = 1
Work hours = 8H per day.
Then, Duration = 5 Days

2. Fixed Duration
Duration is fixed regardless of efforts, resource and work hours.
For example, in the case of a task such as setting a machine, turning on the switch, and then waiting for a certain period of time.

3. Zero Effort
Effort is zero, and duration can be defined regardless of efforts, resource and work hours.
For example, if you have requested a customer and do not use the resources of your team, but you want to set only the duration because it affects the subsequent tasks.

4. Fixed End Date
When the task is delayed and the end date of the task is revised regardless of efforts, resource and work hours.