While implementing jira.permission.* attributes in 3.6, it became apparent that the permissions code would need some refactoring before we could implement 'jira.permission.subtasks.create'. So a hacky alternative, "jira.issue.subtasks-creatable" was left in the code. Unfortunately this cannot be edited through the workflow editor, being an internal (jira.*) property. We should finish the refactoring and implement the proper jira.permission.subtasks.create attribute.