From the JCL Language Reference
126.96.36.199 Block Sizes for Concatenated Data Sets
Concatenated data sets can have different block sizes. In a few cases, the data set with the largest block size must appear first in the concatenation. (Note that you can state a value equal to the largest block size for BLKSIZE on the first DD statement, regardless of what the actual block size of this data set is.) Certain data sets can be concatenated in any order of block size; these are:
Partitioned data sets (PDSs), and partitioned data sets extended (PDSEs) without member names coded on the DD statements.
Sequential data sets that are DASD-resident, tape-resident, or in-stream, and are accessed by QSAM and use system-created buffers.
Sequential data sets that are DASD-resident or in-stream, and are accessed by BSAM.
For these data sets, the BLKSIZE obtained is the largest in the concatenation. Note that this block size can cause invalid attribute combinations when combined with the attributes obtained from the first data set in the concatenation.
If you do not specify a block size, the system can, under certain conditions, determine an optimum block size. For detailed information about system-determined block size, see z/OS DFSMS Using Data Sets.