性价比很高的问题！(100分)

那位大侠能给我详细解释以下的代码的含义：
top:=screen.Height -50;
left:=screen.Width -120;
height:=label1.Height ;
width:=label1.Width;
margin:=(width-clientwidth) div 2;
fullRgn:=createrectrgn(0,0,width,height);
x:=margin;
y:=height-clientheight-margin;
clientrgn:=createrectrgn(x,y,x+clientwidth,y+clientheight);
combinergn(fullrgn,fullrgn,clientrgn,rgn_diff);
x:=x+label1.Left;
y:=y+label1.Top;
buttonrgn:=createrectrgn(x,y,x+label1.Width,y+label1.Height);
combinergn(fullrgn,fullrgn,buttonrgn,rgn_or);
setwindowrgn(handle,fullrgn,true);

感谢！！！

在屏幕右下角出现一个类似foxmail提醒时出现的那个东东

能不能说的详细点？比如这一句
margin:=(width-clientwidth) div 2;是什么意思？
fullRgn:=createrectrgn(0,0,width,height);又是干吗的？
请仔细说说，谢谢

margin:=(width-clientwidth) div 2;<<除法运算这也要问
fullRgn:=createrectrgn(0,0,width,height);建立一个长方型的窗口句柄