When you set a task to fixed work, Microsoft Project does not calculate the work for the tasks or resources. Instead, as you change resource assignments, Microsoft Project calculates duration.
For example, if you assign two painters full-time to paint the walls in 4 days, and then you assign two more full- time painters to the task, Microsoft Project reduces the duration to 2 days. If you adjust the task’s duration, Microsoft Project adjusts each resource’s units. For example, if you now increase the duration from 2 days to 4 days, Microsoft Project sets each resource’s units to 50 percent, so each painter works only half-time.