No Label (NL) Processing

You should specify NL in the FILEDEF command when you expect a tape does not contain any IBM standard tape labels. CMS reads your tape at the time a file is opened and does not open the file if the tape contains a VOL1 label as its first record. If the tape does not contain a VOL1 label, a file is opened and the tape is positioned by using the position parameter (n). For example, if you specify
filedef fileq tap1 nl 2
FILEQ is not opened if the tape on TAP1 (181) has a VOL1 label. If the tape does not have a VOL1 label, FILEQ is opened and the tape is positioned at the second file. If you do not specify a position parameter, the tape is positioned at the first file, (that is, the load point).