...
Every task has its schedule type which is one of the following:
Fixed Work - Set the task to Fixed Work when you want the amount of work to remain constant, regardless of any change in duration or resource units.
Example: Two welders are assigned full-time (100%) to build a gate in 2 days. Work/Effort is 32 hours. If you now increase the duration of the task from 2 days to 4 days, Celoxis will reduce each welder's assignment to 50%, keeping work constant.
Fixed Duration - When you choose to make a task Fixed Duration, the task's duration remains at whatever value you enter regardless of any change in work or resource units.
Example: One welder is assigned full-time (100%) to build a gate in 2 days. Duration of this task is fixed. Thus work is 16 hours. If you now reduce this welder's assignment to 50% due to another commitment, Celoxis will reduce work to 8 hours, keeping duration constant.
Fixed Units - When you choose to make a task Fixed Units, whatever resource units you allocate, that value will remain fixed and the other two elements of the scheduling formula will be affected by changes.
Example: One welder is assigned full-time (100%) to build a gate in 2 days. Units of this task is fixed. Thus work is 16 hours. Now if you increase duration to 4 days, Celoxis will increase work to 32 hours, keeping units the same. By specifying one of these, you can control which of the scheduling formula parameter (See: Automatic Scheduling) has the most influence over the task's scheduling.
...
If a project's start date is in the past, adding a new task today will set its start date to today with a 'SNET' constraint. If a project's start date is today or in future, adding a new task today will set its start date to the project's start with an 'ASAP' constraint. Changing Project dates, will shift all tasks with 'ASAP' or 'ALAP' constraints as appropriate.