Both built-in scripts and custom scripts offer a "Run" option which indicates whether this particular script should be run against each file or only after all files have been downloaded from all sources. Typically, the "Run" option should be set to "Per File"; "Once After All Downloads" is handy when a task needs to do something like zipping multiple files into a single archive.
Setting the option to Per File causes the process to be run once for each file downloaded from all sources, before the file goes out to any destination hosts. If there are no sources, the process will be run once.
Setting the option to Once After All Downloads causes the process to run once, and only once, after all source files have been downloaded, and before any files are uploaded to any destination hosts. The process will be run once even if no files have been downloaded. Such a process can determine whether any files were downloading by looking to see whether the MICacheFiles() function returns an empty string.
Built-in scripts feature an "Edit Parameters" button which will allow operators to select required and optional parameters for the selected script from a drop-down menu.