...

This builds off of How to create multiple issues and is part of release 3.3. This can be used to create subtasks or issues, the example here will be for subtasks. Advanced techniques are used to show:

...

1. Follow the steps similar to How to create multiple issues
2. Determine the field in the original issue that will control the number of issues to be created. For instance, this could be one of the many Substitution variables that produce a comma separated list like original_fixed_versions, role_developers, original_labels_comma_separated, or similar.
3. Determine any include or exclude criteria for subsetting the entries to be considered.
4. Determine if duplication prevention is needed and, if so, determine a condition that could be used to determine if the create should proceed. A JQL query is a convenient way to do this.
5. Configure the post function - see the example.

...

Example - Setting unique values for other fields

Create multiple subtasks on transition that have different assignees discusses some of the techniques that can be used. Provide substitution variables for custom field option values provides the basic enablement for this.