I have had a look at https://support.atlassian.com/jira-software-cloud/docs/automation-actions/#Automationactions-Lookupissues which explains how we can do the bulk processing of jira tickets to send in emails etc. But this only states that we can use system data. When can we or can we expect at all that this lookup issue actions will work with custom field data.
The big use is being able to process data out for emails etc. System data is great but custom fields allows it to be so much richer.