Specifies the duration of this task as working time, not calendar time. 7d means 7 working days, or 7 times 8 hours (assuming default settings), not one week.

A task with a length specification may have resource allocations. Resources are allocated when they are available. There is no guarantee that the task will get any resources allocated. The availability of resources has no impact on the duration of the task. A time slot where none of the specified resources is available is still considered working time, if there is no global vacation and global working hours are defined accordingly.

For the length calculation, the global working hours and the global leaves matter unless the task has shifts assigned. In the latter case the working hours and leaves of the shift apply for the specified period to determine if a slot is working time or not. If a resource has additional working hours defined, it's quite possible that a task with a length of 5d will have an allocated effort larger than 40 hours. Resource working hours only have an impact on whether an allocation is made or not for a particular time slot. They don't effect the resulting duration of the task.

Tasks may not have subtasks if this attribute is used. Setting this attribute will reset the duration, effort and milestone attributes.