This command is used to start an external command defined in a dynamic load library. Please read the details of the command DECLARE SUB if you did not do up to know before reading the details of this command.
The major difference between an external function and an external command is that arguments are passed after they are evaluated to external functions, while arguments are not evaluated for external commands.
External commands are a bit more tedious to implement. On the other hand external commands have more freedom handling their arguments. A command is allowed to evaluate or not to evaluate some of its arguments. It can examine the structure of the expression passed as argument and evaluate it partially at its wish.
From the BASIC programmer point of view external commands are called the same way as user defined functions or external functions.