what is an outline design used for ?

Well, it's just what it says - a plan to give you a general idea before you start putting in details of planting/ hard landscaping etc. It should have the size of the area, the shape either given already or what you decide it should be, the aspect,(which way the area faces) and if there are any features already there - e.g. a mature tree, a wall etc.

The detail comes at the next stage.

Is this a course you are doing?