The Multilevel structure feature is deprecated and no longer supported. It’s unavailable for the users of Extension for Jira Service Management version 6.10.0 and higher.
For more information on adding a custom field, see official Atlassian documentation.
To set configuration:
It is possible to select more than one request type and create structure up to four levels.
Remember, text field can be added only inside the list as a second or third stage.
Available custom fields list displays only these custom field types: checkboxes, radio buttons and text.
A multilevel structure is created and added to the list.