$ZJ[OB] holds the pid of the process created by the last JOB command performed by the current process.

GT.M initializes $ZJOB to zero (0) at process startup. If the JOB command fails to spawn a new job, GT.M sets $ZJOB to zero (0). Note that because of the left to right evaluation order of M, using $ZJOB in the jobparameter string results in using the value created by the last, rather than the current JOB command, which is not likely to match common coding practice.

GT.M does not permit the SET or NEW command to modify $ZJOB.