//JOB8   JOB  ,BOB,MSGLEVEL=(2,0)
//JOBLIB DD   DSNAME=DEPT12.LIB4,DISP=(OLD,PASS)
//STEP1  EXEC PGM=USCAN

These statements indicate that the system is to search the private library DEPT12.LIB4 for the member named USCAN, read the member into storage, and execute the member.

//PROCESS JOB  ,MARY,MSGCLASS=A
//CREATE  EXEC PGM=IEWL
//SYSLMOD DD   DSNAME=&&PARTDS(PROG),UNIT=3390,DISP=(MOD,PASS),
//             SPACE=(1024,(50,20,1))
//GO      EXEC PGM=*.CREATE.SYSLMOD

The EXEC statement named GO contains a backward reference to DD statement SYSLMOD, which defines a library created in the step named CREATE. Program PROG is a member of the partitioned data set &&PARTDS, which is a temporary data set. Step GO executes program PROG. The data set &&PARTDS is deleted at the end of the job.

//JOBC   JOB  ,JOHN,MSGCLASS=H
//STEP2  EXEC PGM=UPDT
//DDA    DD   DSNAME=SYS1.LINKLIB(P40),DISP=OLD
//STEP3  EXEC PGM=*.STEP2.DDA

The EXEC statement named STEP3 contains a backward reference to DD statement DDA, which defines system library SYS1.LINKLIB. Program P40 is a member of SYS1.LINKLIB; STEP3 executes program P40.