Suppose there is a sub process in a main process. When we execute main process multiple times then it will generate multiple instances for main process as well as sub process. So how to identify which sub process instance belongs to which main process instance?
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For each process model you can use an expression to define the display name https://docs.appian.com/suite/help/21.2/process-model-object.html#general-tab
For your case, you can pass in the parent process ID or another identifier and display it in the sub-process display name
Also to note, you can still retrieve the parent process ID without passing it to the sub as this is available natively in process reports by defining a column as "=parent_process_id()", via Process Metrics.
Agree with both comments above. Also remember you can always go File > Open Parent Process. This obviously only helps when manually navigating the process models but very handy when responding to an incident.
Note (in case it matters for some reason) that if the parent process instance is archived or deleted before the subprocess is, then =parent_process_id() will return blank. If it's important to have it persistently though, a node in the model could store the value into a PV (though I wouldn't do this unless there's a defined need for it).