Scaled can be used to specify scaled coordinates in any two- or three-dimensional graphics primitive.
You can use Scaled to represent objects that occupy a fixed region in a plot, independent of the specific range of coordinates in the plot.
Scaled refers only to the region of a plot defined by its PlotRange and PlotRangePadding. It does not take account of elements such as tick labels, or of margins introduced by ImageMargins.
Scaled can be used not only for coordinates, but also for scalar length measures such as radii.
In 2D graphics, Scaled[r] is taken to be relative to the width of the whole graphic. In 3D graphics, it is taken to be relative to the diagonal of the 3D bounding box.