When does a book go from YA to Upper YA?

[Kinsley]

In your opinion, what makes a book Upper YA? I know what NA is but what is Upper YA? Would love to see what others think!

[Chloe]

I feel like upper YA is when sex is involved, not explicitly since it's still YA but more closed door style. Standard YA is to me is completely clean.
Like Shatter Me would be upper YA but a Kasie West book would be standard YA