I think GP was referring to the Unix System Services, which give you a Unix (z/OS is a certified UNIX operating system!) environment. If you are interacting with the machine on a 3270 session, it'll be any of a number of command-line or menu-driven environments (3270's are screen oriented terminals that send entire pages back and forth). The fun part is that z/OS is almost the least Unix-like operating system imaginable - even the concept of files and directories is alien to it.