I was trying to resolve problems with a started task, and the output kept disappearing.

The JES2 class had been defined so it purged the JES2 output if the job ended cleanly

$DJOBCLASS('STC'),OUTDISP 

gave me

$HASP837 JOBCLASS(STC)       OUTDISP=(PURGE,HOLD) 

This says, if the job ends normally then purge the JES2 related datasets.

I needed

$T JOBCLASS(STC),OUTDISP=(Keep,keep)

to keep the JES2 related datasets around. This command should be remembered across warm starts of JES2.

With $HASP837 JOBCLASS(STC) OUTDISP=(KEEP,HOLD) the spool files in the output are

DDNAME   StepName ProcStep C Dest 
JESMSGLG JES2              K LOCAL
JESJCL   JES2              K LOCAL
JESYSMSG JES2              K LOCAL
CFGPRINT TCPIP             K LOCAL
SYSOUT   TCPIP             H LOCAL
SYSERROR TCPIP             K LOCAL

With $HASP837 JOBCLASS(STC) OUTDISP=(PURGE,HOLD) the spool files in the output are

DDNAME   StepName ProcStep C Dest 
SYSOUT   TCPIP             H LOCAL
SYSERROR TCPIP             K LOCAL
CFGPRINT TCPIP             K LOCAL

where the JES* files have been purged.

If your job does not provide any output, then when the JES* files are purged, there will be no trace of the job in the spool.