What are the different categories for parenthetical asides that are set aside by commas, dashes or parentheses? You have nonrestrictive descriptors which function syntactically as modifiers, interjections (such as 'yeah', 'oh', etc.) that have no syntax function, and...
And what else? Are there other categories I'm missing?
Take this sentence, for instance: "Becoming a great writer—or even a good writer—requires practise and patience."
The phrase bracketed in dashes is a coordinate completer of the gerund 'Becoming', which itself is the sentence's subject... Correct? So how is the phrase in dashes functioning in terms of the syntax? I'm not looking for an answer along the lines, "It's just a nonessential element..." How does that enclosed phrase relate to the remainder of the sentence?
If there's any source that teaches the logic of nonessential elements in the meticulous detail I'm looking for, I'd appreciate a link/reference. I have struggled with understanding them for months now! I don't know why I can't get my head around them. I have studied syntax in detail, learning the logic of the language, the roles of each word category, etc., etc., but this single issue has driven me crazy! Anyway, apologies for this mini-rant.
Thanks for any responses!