Attached you will find a spreadsheet, with all the possible alert notification subscription variables that I am aware of. In this spreadsheet, I link these to the same values in the Alert table of the DB, the alert view of the DB, the Console alert view, the SDK (Get-Alert), and lastly the new R2 Connector Key pairs.
My thought was to better understand each data property type of an alert, and what you can managed, from each area. Hope this is beneficial.
Most of these are listed at my other blog post on alert description and notification variables: https://kevinholman.com/2007/12/12/adding-custom-information-to-alert-descriptions-and-notifications/
Here is a sample shot of the spreadsheet tool: