JES2 accepts jobs (in the form of an input stream) from input devices such as card readers, remote terminals, or other programs. Input streams can also come from other nodes in a job entry network and from internal readers. An internal reader is a program that other programs can use to submit jobs, control statements, and send commands to JES2. Any job running in z/OS® can use an internal reader to pass an input stream to JES2, and JES2 can receive multiple jobs simultaneously through multiple internal readers.

z/OS uses internal readers, allocated during system initialization, to pass to JES2 the job control language (JCL) for started tasks, START and MOUNT commands, and TSO LOGON requests.

The system programmer defines internal readers used to process all batch jobs other than STCs and TSO requests. JES2 initialization statements define these internal readers which JES2 also allocates during its initialization processing. The internal readers for batch jobs can be used for STCs and TSO requests, if not processing jobs.

As JES2 reads the input stream, it assigns a job identifier to each job and places each job's JCL, optional JES2 control statements, and SYSIN data onto DASD data sets called spool data sets. JES2 then selects jobs from the spool data sets for processing and subsequent running. See Scope of control and configurations for an explanation of spool data sets and the concept of spooling.