Jira uses smart values to access and manipulate Jira data. Here's how to inspect an actual issue and see its data structure with all of the fields.
Simply visit the following URL to see a JSON representation of your issue:
https://<yourinstanceurl>/rest/api/2/issue/<issuekey>?expand=names
Smart Fields in Jira use the following syntax: {{smartField}}.
To test what a smart value returns, use the Manual trigger with Log action. The result displays in the audit log.
- Jira Docs: Find the smart Value for a field