COBOL Manual for TNS/E Programs (H06.08+, J06.03+)

SELECT clause
OPTIONAL
makes the file optional, which means that an OPEN statement with an INPUT or EXTEND
phrase can open the file whether or not the file exists. If the file exists, its I-O status code
is “00”; if not, its I-O status code is “05”. OPTIONAL does not affect the OPEN statement
with an OUTPUT phrase.
When you open a nonexistent optional file for input, the first READ statement for that file
uses the AT END option (or USE procedure if the READ statement has no AT END phrase).
file-name
is the COBOL file-name (the file-name in a file description entry).
ASSIGN clause
associates file-name with system-file-name or define-name-literal. Only the
first system-file-name or define-name-literal has meaning. The compiler ignores
subsequent names and literals and issues a warning.
system-file-name
is the name of a file that the file system recognizes or one of the special operating system
file names described in System-Names (page 65). If system-file-name does not begin
with a dollar sign ($), backward slash (\), or number sign (#), then it must be enclosed in
Input-Output Section 119