PromptPointOptions 类的疑惑，问题在代码第51行

/*代码来自CAD.net开发人员手册,
本例提示用户输入五个点，然后根据输入的点创建多段线。该多段线是闭合的，所形成的面积显示在消息框中。因为多段线不需要添加到块中，所以在命令结束前需要删除它。问题在代码第51行*/
using Autodesk..ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
[CommandMethod("CalculateDefinedArea")]
public static void CalculateDefinedArea()
{
  // 提示用户输入 5 个点    Prompt the user for 5 points
  Document acDoc = Application.DocumentManager.MdiActiveDocument;
  PromptPointResult pPtRes;
  Point2dCollection colPt = new Point2dCollection();
  PromptPointOptions pPtOpts = new PromptPointOptions("");
  // 提示输入第一个点   Prompt for the first point
  pPtOpts.Message = "\nSpecify first point: ";
  pPtRes = acDoc.Editor.GetPoint(pPtOpts);
  colPt.Add(new Point2d(pPtRes.Value.X, pPtRes.Value.Y));
  // 如果用户按了 ESC 键或取消了命令就退出   Exit if the user presses ESC or cancels the command
  if (pPtRes.Status == PromptStatus.Cancel) return;
  int nCounter = 1;
  while (nCounter <= 4)
  {
      // 提示指定下一个点   Prompt for the next points
      switch(nCounter)
      {
          case 1:
              pPtOpts.Message = "\nSpecify second point: ";
              break;
          case 2:
              pPtOpts.Message = "\nSpecify third point: ";
              break;
          case 3:
              pPtOpts.Message = "\nSpecify fourth point: ";
              break;
          case 4:
              pPtOpts.Message = "\nSpecify fifth point: ";
              break;
      }
      // 使用前一个点作为基点   Use the previous point as the base point
      pPtOpts.UseBasePoint = true;
      pPtOpts.BasePoint = pPtRes.Value;
     //问题在这里，UseBasePoint 和BasePoint 属性指的是什么？请指教。
      pPtRes = acDoc.Editor.GetPoint(pPtOpts);
      colPt.Add(new Point2d(pPtRes.Value.X, pPtRes.Value.Y));
      if (pPtRes.Status == PromptStatus.Cancel) return;
      // 递增计数器    Increment the counter
      nCounter = nCounter + 1;
  }
  // 用5个点创建多段线   Create a polyline with 5 points
  using (Polyline acPoly = new Polyline())
  {
      acPoly.AddVertexAt(0, colPt[0], 0, 0, 0);
      acPoly.AddVertexAt(1, colPt[1], 0, 0, 0);
      acPoly.AddVertexAt(2, colPt[2], 0, 0, 0);
      acPoly.AddVertexAt(3, colPt[3], 0, 0, 0);
      acPoly.AddVertexAt(4, colPt[4], 0, 0, 0);
      // 闭合多段线   Close the polyline
      acPoly.Closed = true;
      // 查询多段线的面积   Query the area of the polyline
      Application.ShowAlertDialog("Area of polyline: " +
                                  acPoly.Area.ToString());
      // 销毁多段线   Dispose of the polyline
  }
}