Spool

The spool is where output goes. When a batch job runs, everything it prints, from system messages and program output to reports and error text, is written to the spool rather than straight to a printer or a screen. It stays there until someone reads it, prints it, or purges it.

The name is an old acronym, Simultaneous Peripheral Operations On-Line, from the days when the alternative was queuing for a physical printer. The concept survived because it is useful: work does not have to wait for a device to be free, and output can be reviewed long after the job finished.

You browse the spool through SDSF. Each job that has run leaves a set of output datasets there, typically including the JCL as the system interpreted it, the messages the system produced, and whatever your program wrote.

Spool space is finite and shared, which is why sites purge old output on a schedule. If you need to keep something, copy it to a dataset rather than leaving it on the spool.