You cannot mix FB and VB formatted datasets under the SYSEXEC DD. The customer issued the TSO ISRDDN and received this warning for SYSEXEC concatenation:
Warning: ISRDDN has detected that one or more concatenations, including DD name SYSEXEC contain mixed record formats, organizations, or fixed record lengths. Reading from allocations containing different types of data sets can cause I/O errors, ABENDs, or other unpredictable results. For further information, see the z/OS documentation on using data sets. Enter CHECK OFF to disable this check.
Datasets in the SYSEXEC concatenation:
8906 255 VB PO > SYSEXEC STC1.REXXLIB
27920 80 FB PO > ISP.SISPEXEC 27920 80 FB PO > ISF.SISFEXEC
27920 80 FB PO > SYS1.SABXEXEC
27920 80 FB PO > SYS.OPSMVS.LSYS.REXX
8906 255 VB PO > STC1XZY.REXXLIB
You cannot mix FB and VB formatted datasets under the SYSEXEC DD.