I was trying in an expression rule from a processId to get the data that is inside the process variable, that is inside the process model.It is possible?
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Yes, you can create a process report by that said process model. Then add the process variable as one of the report column. Now, using process Id you can query the process and get the process variable data.
You can go through this documentation for configuration steps : Configuring Process Reports
Yes. You can write a Process Report (see: https://docs.appian.com/suite/help/21.2/Configuring_Process_Reports.html) which can get access to the process variable data. You can then invoke the Process Report from within an Expression Rule to get the data you've specified.
The steps Stewart and Harsha outlined above already cover this. What exactly is still not working for you?
You can create a custom Process Report that can be filtered by one or more ProcessIds, and that report can reference one or more pv!s from the target process model. The link I included above outlines how this is done.