Using previously un-used heading levels would be more suitable for situations where you want to maintain two separate numbered lists that both make use of the "magic" properties of the built-in headings; for example, this would be the case when you set up appendix numbering as discussed in the article at https://shaunakelly.com/word/numbering/numberingappendixes.html.
In the Adobe InDesign document I’m working on here we actually have two master pages, an ‘A’ master and a ‘B’ master so I’m going to select both the left and right hand page frames with the special character in on my ‘A’ master and go to Edit>Copy. I’m then going to double click on my ‘B’ master page and go to Edit>Paste in place. this will then copy the special character and position it in exactly the same place as it appears on my ‘A’ master. numbering in publisher