Inside of Vensim, external function are referred to exactly the same way built in functions are. You simply call the function with the number of arguments you want. If the external functions you are using have user loops, vector arguments, or are marked as modifying values things get more complicated.
Arguments are always passed in the order Literal, Lookup, Vector, Value. You can not pass arguments in any other order. For functions that are marked as self looping an additional Vector argument is added before any of the declared arguments. Because of this a self looping function with 4 arguments in Vensim will be called with 5 arguments and the first of these will be a VECTOR_ARG for the left hand side variables.
