LUD:局部变量，在定义的程序内使用
PUD:局部变量，通过设定参数，在主程序内定义后在子程序内也可使用
GUD:全部变量，所有程序内可使用

下面是原文
Local variables (LUD) are only valid in the program where they are defined.
Global variables (GUD) are valid in all programs.
Machine data are used to redefine the local user variables (LUD) defined in the main
program as program-global user variables (PUD).