The MODS statement is used to tell DFSORT to call user exits. In this case, DFSORT will call an E15 exit named S0XRSX15 and an E35 exit named S0XRSX35, both of which reside in the library indicated by the MODLIB DD.
For complete information on the use of the MODS statement with DFSORT, see:
Frank Yaeger - DFSORT Development Team (IBM) - yaeger@us.ibm.com Specialties: JOINKEYS, FINDREP, WHEN=GROUP, ICETOOL, Symbols, Migration => DFSORT/MVS is on the Web at http://www.ibm.com/storage/dfsort