Hidden process variable is hide from reports other than that is there any other scenarios where we can use those variables?
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Hidden variables are really useful in Appian if you want to have processes that have a low memory footprint. You can consider them in some way as temporary variables that will not be available when the process ends and you want to see the variables that have been used, or check process history
In this way, even if you have very heavy variables in the process instance, the weight once it has finished will be small, which will prevent the system from having a high consumption of resources by processes that have already finished.