Put Simply:
Currently in JIRA, it is possible to add hierarchy levels other than the six defined in PDM—Stage, Sub-Stage, Activity, Deliverable, Task, and Sub-Task—which deviates from the PDM format. Please implement a restriction that prohibits the creation of any hierarchy levels other than those specified in PDM.
Thank you for this submission, Kazuki. This item will be reviewed by the Product team to determine if this will be implemented.