User Exits
A user exit is an internal Open C and C API program with a unique entry point that occurs at a specific location within Unigraphics NX. The argument list for each function is identical to ufusr. You can obtain full details on user exits from the Reference Help or the header file uf_exit.h. Each user exit has a unique environment variable that points to the image that is to be executed. When this environment variable is defined and you select the correct menu option within UG, your internal Open C and C API Program is executed. For example, there is a user exit that occurs at File—

能不能搞点中文的，我的英文很菜的！

简单一点说：第一个入口是用菜单调用的；第二个可以做为一个函数被别的程序调用；第三个是用user-execute function那里调用的（函数名为ufusr）。