Is UserErrors a magic string that somehow connects the message dictionary with $system.Status?
I don't see the string mentioned in https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls... nor anywhere in the documentation.

I was able to google https://community.intersystems.com/post/registering-new-error-code-and-e... that seems identical to this post.

About your second point: If you are new to git I recommend studying a git tutorial to get the basics and terminology right. When you know how git works most of those menu options will become self-evident as they match standard git operations. The internet  has plenty of git material but one "authoritative" source and a good starting point is https://git-scm.com/book/en/v2