After the execution of any file I/O verb (CLOSE, DELETE, OPEN, READ, REWRITE, START, UNLOCK, or WRITE), information about the I/O is available in the form of a status number. This number is placed in the FILE STATUS variable for the appropriate file if one has been declared.
The format of the status value is a 2-digit number. The first digit indicates a general classification of the status, while the second digit provides detail information. Any file status whose first digit is `0>' is considered a successful I/O. Any non-zero initial digit indicates failure.
Book 4, Appendix J covers the exact values returned for each I/O condition.