This is the documentation of JSU for Jira Cloud. If you are using JSU on Jira Server or on Jira Data Center, you can find the documentation here.
The 'Update any Issue Field' post-function updates any field to a specified value, after a transition has been completed. This can be a system- or a custom field.
The field can be on the issue in transition(within same issue) or on a related issue, like a sub-task, a linked issue or an issue within an Epic (during the transition on the Epic).
You must specify the field and the desired value. For example:
Please make sure that the value you enter is valid for the datatype of the selected field. Also verify, that the context configuration for the project using this workflow will allow to modify the selected field.
Typically you will use text or numbers as value.
See the above example of a Story changing the Epic Status of its Epic.
Or whenever a parent issue is set to 'In Progress' the assignee of all its sub-tasks could be set to the current user (the one changing the status on the parent).
A developer has fixed a Bug. He proceeds in the Jira workflow to the status 'Resolved' (this might be triggered from his code pushed to Bitbucket). The 'Update any Issue Field' now adds a label 'testing-required'.
This would be a very light weight solution. There are also cases when you need a more complex solution. Have a look at Testing and Fixing Bugs.
For more information on how to configure a post-function in Jira, see the Jira documentation.