The file system recognizes two general classes of files:
user-defined files, which you or other users define, create, and
make available for your own purposes
system-defined files, which the file system defines and makes
available to all users to indicate standard input/output
devices
These files are distinguished by the file names and other
descriptors (such as group or account names) that reference them,
as discussed below. You may use both the file name and descriptors,
in combination, as either formal designators within your programs
or as actual designators that identify the file to the system. Generally,
however, most programmers use only arbitrary names as formal designators,
and then equate them to appropriate actual file designators at run
time. In such cases, the formal designators (user file names) contain
from one to eight alphanumeric characters, beginning with a letter;
the actual designators include a user or system file name, optionally
followed by a group name, account name, and/or security lockword,
all separated by appropriate delimiters. This technique facilitates
maximum flexibility with respect to file references.