PL/I supports FIXED BINARY (and the length field of VARYING strings) to be stored in NATIVE (default and native to the machine) order, or in BIGENDIAN order, which may also be the native order of the machine. When BIGENDIAN order is used on LITTLEENDIAN machines, extra instructions need to be generated when the compiler stores or fetches the contents of FIXED BINARY fields.
Use the -bigendian compiler option when mainframe files are being accessed and/or you have FIXED BINARY(31) overlaid on POINTER for arithmetic purposes and/or you are relying on the order of bytes within a FIXED BINARY field.