Part of how it works – on Unix, command arguments are just a list of strings, and any option parsing (etc.) has to be done by the code of each command. By contrast, on IBM i, there is a declarative language used to define the syntax of the command (allowed options/arguments, what values are allowed for each, etc.) The declarative command definition is compiled into binary form and installed into the system. When you run a command, the operating system uses the installed command definition to parse and validate the parameters and marshal them into a memory block. If the parameters are invalid, then the OS produces the error and the command executable never even starts. If the parameters are valid, the memory block gets passed to the command's code. These same declarative command declarations are what gets used to generate the fill-in form UI.