Nunjucks in JMWE for Jira Cloud is used to insert information in
Set field value to
Subject, HTML body, Text body
Recipientsfrom the script in
Users from templateand
Groups from template
JQL search expression
You can insert issue and transition information into the value using the templating features available in Nunjucks. Some basic templating features are described in this document, and more are documented on the Nunjucks website.
You might also want to look at Accessing the details of an issue or a transition in Nunjucks to know how to access the details of an issue or a transition.
See here for more templating features available in Nunjucks. You might also want to see here for use cases with Nunjucks annotations.