A resource that determines the duration of the activity to which it is assigned. P6 Professional automatically calculates the activity duration based on the quantity to complete and the units per timeperiod of the driving resource.
Defining resources as driving, by marking the Drive Activity Dates by Default checkbox for new resource assignments in the Projects window, Project Details, Resources tab, only establishes its default status; you can change it for any specific assignment.
If an activity has more than one driving resource, the resource that takes the longest to complete determines the duration of the activity.