While I was able to find a definition for the priorities of customer problem incidents in sap note 67739 and that "SAP HotNews are priority 1 (very high priority) SAP Notes." that tell me "how to resolve or avoid problems that can cause the SAP system to shut down or lose data." https://support.sap.com/kb-incidents/notifications/help.html
And also managed to find out how SAP defines the priority of SAP security notes https://support.sap.com/en/my-support/knowledge-base/security-notes-news/security-notes-faqs.html
I could not find any definition for the priorities of the SAP Notes in general, or how their priority is determined.