Exactly what Justin said. To provide a screenshot:
You can see that all IM processes will have a process id prefixed with "IM-" and will then be followed by at least six digits. If the number would be less than six, it's padded with "0".
For what it's worth, the number that will be used is stored in the ReportProcessGenerator table on your DB. It just looks at the last used number by type of process ("IM-", "CM-", etc.) and increments by one.