Use of the Global Namespace Aliases

Hi,

  Here is code for cropping image using Js. I want to know that why we are using such type of name space 

using SD = System.Drawing;

SD is worked here like object but we can  access directly properties if we added namespace. But in this code I don't understand how SD worked. 

using (SD.Image OriginalImage = SD.Image.FromFile(Img))

Whole code is as following.... 

using System.IO;
using SD = System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D; 

static byte[] Crop(string Img, int Width, int Height, int X, int Y)
{
  try
  {
    using (SD.Image OriginalImage = SD.Image.FromFile(Img))
    {
      using (SD.Bitmap bmp = new SD.Bitmap(Width, Height))
      {
        bmp.SetResolution(OriginalImage.HorizontalResolution, OriginalImage.VerticalResolution);

        using (SD.Graphics Graphic = SD.Graphics.FromImage(bmp))
        {
          Graphic.SmoothingMode = SmoothingMode.AntiAlias;
          Graphic.InterpolationMode = InterpolationMode.HighQualityBicubic;
          Graphic.PixelOffsetMode = PixelOffsetMode.HighQuality;
          Graphic.DrawImage(OriginalImage, new SD.Rectangle(0, 0, Width, Height), X, Y, Width, Height, SD.GraphicsUnit.Pixel);
          MemoryStream ms = new MemoryStream();
          bmp.Save(ms, OriginalImage.RawFormat);
          return ms.GetBuffer();
        }
      }
    }
  }
  catch (Exception Ex)
  {
    throw (Ex);
  }
}