Each process you start has a unique identifier known as the Track ID. It’s in the middle of the toolbar:
Its auto generated, but if you have a more appropriate number, you can override it. Simply uncheck the ‘auto generate’ check, then enter the number. You can only do this when starting a process, and its only accepted if it hasn’t been used before.
This number can be used to check the process status in reports – on external form submission; for example, users are shown this number so they may track the progress of their submissions.