Once an ObjectARX application has defined its external functions (with calls to acedDefun()), the functions can be called by the AutoLISP user and by AutoLISP programs and functions as if they were built-in or user-defined AutoLISP functions. An external function can be passed AutoLISP values and variables, and can return a value to the AutoLISP expression that calls it. Some restrictions apply and are described in this section.