When a DD statement contains a UNIT=AFF=ddname parameter, the DD statement referenced in the AFF subparameter must be defined earlier in the job step; otherwise, the system treats the DD statement containing UNIT=AFF as a DD DUMMY statement.

   //STEP  EXEC  PGM=TKM
   //DD1   DD    DDNAME=DD5
   //DD2   DD    DSNAME=A,DISP=OLD
   //DD3   DD    DSNAME=C,DISP=SHR,UNIT=AFF=DD1
   //DD5   DD    DSNAME=B,DISP=SHR

DD3 requests unit affinity to DD1. Although DD1 occurs earlier in the job step than DD3, it refers to DD5 that is located after DD3. Because DD1 is not completely defined, the system treats DD3 as a dummy statement.