Poetry

And I would say that Poetry is, definitively, Non-Fiction. While we tend to think of Non-Fiction as "true stuff" (or at least supposedly true), that's not really the category. The category is just "not Fiction," and that includes the Histories and Biographies we think of first, but also poetry, stage plays, and whatever other monographs you might have that just aren't Fiction.

To use James' vehicular example, our categories are "cars" and "not-cars," and we keep forgetting that motorcycles are included with Semi-trucks because they're all not-cars. (Whether that's a good way to categorize everything is maybe a different discussion.)

And on the other hand, personally I wouldn't use a "General Non-Fiction" subject on poetry because poetry is a specialized type of non-fiction, not generally what people are thinking of when they ask for Non-Fiction. I would use "General Non-Fiction" more on the not-super-academic Histories and Biographies, Contemporary Issues, and maybe some of the more general Political titles, and trust that if someone wants poetry, stage plays, more academic texts, or other more niche Non-Fiction titles, they'll ask for those subjects more specifically. (Similarly, personally I wouldn't put "General Fiction" on every Fiction title, just the "airport reads" and book club picks--not high fantasy, paranormal romance, horror, erotica, etc. It's certainly not the only way to approach the "general" subjects, but that's my thought on it.)