In Project Management, each resource on a task may be assigned different hours of effort for that task. The total effort for the task is the sum of the scheduled effort values for each resource. For example, if Fred is assigned to work 12 hours on a task, and Barbara is assigned to work 8 hours on the same task, then the total task effort would be (12 + 8 =) 20 hours.
Microsoft Project uses resource units to calculate actual work. Following the same example, if both Fred and Barbara are full-time resources in Microsoft Project, then they might have 60 and 40 resource units assigned on the task, respectively (depending on the task's duration). But if Barbara is only a half-time resource in Microsoft Project, her resource units on the task would double (to 80), representing the same amount of work, which now takes a larger percentage of her capacity.
In Project Management-controlled mode and shared control mode, during synchronization, the hours of effort for each assigned resource on a Project Management task is automatically converted by Microsoft Project into the appropriate resource units for the assignment. If the resources involved are full-time resources in Microsoft Project, then their respective resource units for each task is proportional to their respective assigned effort values on the corresponding Project Management task.
In Microsoft Project-controlled mode, during synchronization, the resource units for each task assignment are automatically converted by Microsoft Project into hours of effort for the assigned resource on a Project Management task.