There are process ID numbers, and job numbers. How are they different? Job numbers are never re-used between reboots, while process ID numbers can be re-used once a program terminates. Process ID numbers must be unique on a system. Job number are specific to the user. So there can only be one process ID number of 57 on a system, but each user could have a job number of 2. Both are actually the same, and a job's number exactly matches the process ID number of that job. Process numbers always start at 100, while job numbers start at 1. This is because users are more likely to type job numbers and they wanted them to be shorter.