Effect of DCB=dsname parameter
If the DCB parameter refers to a cataloged data set, the system obtains the volume sequence number from the label of the data set, unless the volume sequence number is coded on the DD statement.
Thus, for the following DD statement, even though DISP=MOD is specified,
the system positions the read/write mechanism after the last record
on the volume specified in the volume sequence number in the label;
this volume may or may not be the last volume.
//DD1 DD DSNAME=MULTI1,DISP=MOD,DCB=CATDD
To control which volume is processed, code a volume sequence number.
//DD2 DD DSNAME=MULTI2,DISP=MOD,DCB=CATDD,VOLUME=(,,2)