In MVS, a file is called a data set, and the two terms are used interchangeably in this book. The z/OS NFS server presents information to you in the form of a UNIX (or AIX) file, though the information is actually stored on MVS-owned DASD in the form of an MVS data set.

The files for a computer system are organized into a file system. The UNIX environment uses a file system that is a hierarchy of directories. MVS, however, uses a non-hierarchical file system in which groups of data sets are referred to by specifying a high-level qualifier.

The MVS high-level qualifier can include the first (leftmost) qualifier of data sets, or the first and second qualifiers, or the first, second, and third qualifiers, and so on. For example, SMITH is the high-level qualifier for the files named SMITH.TEST.DATA and SMITH.PROJ7.SCHED, while SMITH.TEST is the high-level qualifier of SMITH.TEST.DATA and SMITH.TEST.DOCS.