2DModel space is where you do your drawing at full scale e.g. 1:1. You do not normally draw your models in any other scale than 1:1. If you were to draw the Skylab you draw it at full scale in Model space and then plot/print it at the appropriate scale in paper space, this can be A4 to A1 paper size at any scale that will fit. You can also print magnified detail areas of your drawing or part drawing at larger scales.