[whpalmer4]

If you look at the inspector for a task (Task:Information), you'll see a little icon looking like a couple of people from shoulders up next to either the Duration or Effort field. That indicates that the value next to the icon is the one that will be recomputed if you change the resources alloted to the task. If the task is sweeping the driveway, you would put the icon next to the Duration field, because more people out there sweeping would presumably get the job done sooner (though Effort would remain constant). If the task is gestating a baby, you'd put the icon next to the Effort field, because the Duration is going to be fairly constant no matter how many women you assign :-)

Does that answer your question?