JES2 uses a converter program to analyze each job's JCL statements. The converter takes the job's JCL, merges it with JCL from a procedure library (such as SYS1.PROCLIB), and converts the composite JCL into converter/interpreter text that both JES2 and the job scheduler functions of z/OS® can recognize. JES2 then stores the converter/interpreter text on the spool data set. If JES2 detects any JCL errors, JES2 issues messages, and the job is queued for output processing rather than run. If there are no errors, JES2 queues the job for execution. JES2 supports multiple converters; therefore, jobs may not always be processed in a first-in-first-out (FIFO) order. When work load manager (WLM) batch management is in use, JES2 queues the job according to its arrival time.