Project defines any time period in which a resource has an Assignment Units value greater than its Max Units value as an overallocation . This means that if two assignments for a resource overlap by even one minute, the resource will be marked as overallocated if the unit's values for the assignments add up to more than the resource's Max Units field. In the following example, Steve has a Max Units value of 100%. You can see that for the 11:00 hour he is assigned to work one hour on two tasks . This means that the sum of the two Assignment Units fields is 200%, which means that he is 200% allocated for the hour of 11:00.
Notice that when the view is set to show days, the display of hours can be confusing. The day shows a total of eight hours. At first glance, this overallocation looks false. The resource's calendar says he can work eight hours in a day. However, because the tasks overlap as in the preceding example, the resource truly is overallocated. New users are commonly confused by these seemingly false overallocations. That's why it's nice to have the Peak field inserted into your Resource Usage view.
Another important point to remember is that overallocations can occur when the Actual Work values entered for a resource exceed its Max Units value. If you have a resource that worked nine hours of actual work in a day, the resource will show an overallocation for that day.