For Input Files on ANSI or IBM Standard Labeled Tapes:
If you have specified a VOLID parameter on your FILEDEF or LABELDEF
command for a file, the VOL1 label on the tape is checked whenever
the file on that tape is opened. This is to ensure that the correct
volume has been mounted. If the volume ID is specified on both LABELDEF
and FILEDEF, the more recent specification is used. If no volume
ID is specified, the VOL1 label is not checked.
After checking the volid, the tape is positioned according to
the positional parameters specified on the FILEDEF command. The positional
parameters include n, DISP MOD, and LEAVE;
the default is the first file on the tape.
HDR1 labels are checked at open time. This includes checking
the file identifier (FID) or any other information specified in your
LABELDEF command to make sure the correct file is accessed. If you
do not explicitly specify a field for input, the field is not checked.
HDR2 labels are processed, if they exist, to determine certain
data set characteristics of the file such as record format and block
length.
If specified, user header labels (UHLn) are processed. You must
provide a routine to do this.
For read backward processing, data management uses the data set
trailer labels (EOF1 and EOF2) as header labels and vice versa. EOF1
labels are processed before EOF2 labels.