When DCB parameters RECFM, LRECL, and BLKSIZE are not available from the output data set, the IEBGENER program copies these values from the input data set. One or more of the three DCB parameters are copied. See the z/OS DFSMSdfp Utilities for a more detailed table.
If you want to change the output RECFM, BLKSIZE, and LRECL, always specify the RECFM (except for U) and the BLKSIZE on the output DD statement. Also, the LRECL must be present on the output DD statement if editing is required.
DFSMSdfp