Licensing allows for a certain number of concurrent processes. When JetPCL runs, it looks to see if there are other JetPCL processes running. If the maximum number of processes are already running, JetPCL will wait according to the -wait switch setting. If an existing JetPCL process ends then the waiting JetPCL process will continue. By default the -wait switch will wait 3 minutes. You can change the wait time to 5 minutes using -wait:TimeOut=5; for example.
However, if JetPCL is stopped abnormally it is possible for the concurrent process counter to become inaccurate. In this case you need to reset the concurrent process counter using the -wait:Reset switch. This will reset the concurrent process counter. This switch should not be used when other processes are active. It will cause any currently running JetPCL processes to fail. It should be run when no JetPCL processes are currently active. See also, ExitCode 8.