When you assign a resource to a task, you can specify the percentage of the resource’s total daily working hours that the resource will work on the task. That percentage is known as resource units. If you don’t specify resource units, Microsoft Project assumes you want the resource to work full-time (100 percent) on the task (unless it’s a part-time resource).
Assigning a resource at 100 percent units means you want the resource to give all available working hours to the task. If one resource is scheduled to work a total of 6 hours per day, then 100 percent units is equal to 6 hours. For a resource scheduled to work an 8-hour day, 100 percent units is equal to 8 hours (and 25 percent units = 2 hours, 125 percent units = 10 hours, and so on).
With effort-driven scheduling, the more resources you assign to a task, the shorter the duration becomes. In the Gantt Chart view, the Gantt bars decrease in length.
If you want to see how these different factors — link type, duration, resource calendars, and resource units — affect the schedule of a task in your project, you can vary each factor in Microsoft Project until the task is scheduled exactly the way you want it.