FTP clients typically have an auto-detect mode, so why might you want to know this?
Let’s say you’re SSH-ed into one machine and want to get a file from another machine. You won’t have the luxury of using an FTP client to get that file. You’re going to have to use ftp via command line, so not knowing which mode to get the file in will result in corrupt files! (Don’t worry if you discover your file is corrupt, just get the file again in the other mode!)