BSAM Block Size with Like Concatenation

After BSAM OPEN has merged the data set characteristics from the label to the JFCB and the DCB or DCBE, and before it calls your DCB OPEN exit routine, OPEN tries to search later DD statements to see if BSAM should use a larger maximum block size. OPEN searches only if you have enabled a larger block size. A larger block size is enabled if all three of the following conditions are true:

BLKSIZE is not coded in the DCB or DCBE before OPEN or in the first JFCB. Each data set is represented by a JFCB.
RECFM (record format) in the DCB specifies format-U or blocked records. Any data set can be fixed-standard blocked.
You did not set on DCBOFPPC (X'08' in DCBOFLGS, which is at +48 in the DCB). This is the unlike attributes bit.

If you have enabled a larger block size, OPEN searches later concatenated data sets for the largest acceptable block size and stores it in the DCB or DCBE. A block size is acceptable if it comes from a source that does not also have a RECFM or LRECL inconsistent with the RECFM or LRECL already in the DCB.