PDF2XL CLI Command Line Interface allows control over many of the application's settings, and can even be used to run it in Silent Mode, in which the application will load a file and a layout for it and will convert it into an indicated target format without any user interface.
But that is the most extreme form of usage for the command line interface; plenty of other actions can be performed from the command line, such as loading a file automatically when the application is started.
Following this overview, you can look up the different CLI commands, read about the Silent Mode, or view the CLI Examples. If the command line contains errors, PDF2XL CLI will NOT start, and an error code will be returned. A more detailed error message can be found in the standard error output (see the silent conversion example for a way to redirect the standard error output into a file).