Units determine the allocation of a resource to a task. At 100 percent units, a resource is working full-time; at 50 percent units, the resource is working half-time; and so on. By default, units are fixed, meaning allocation remains at the unit value you set. As you change resource assignments, Microsoft Project adjusts the duration. For example, say you assign two full-time painters to paint the walls in 4 days. If you then assign two more full-time painters, Microsoft Project reduces the duration to 2 days.
If you adjust the work for individual resources, Microsoft Project recalculates the total work for the task.