What does fn choose<'a>(x: &'a str, y: &'a str) -> &'a str mean?

What does fn choose<'a>(x: &'a str, y: &'a str) -> &'a str mean?

1. Both inputs must be valid for a caller-chosen region in which the result is used.
2. Both owners must be created and destroyed in the same lexical block.
3. The annotation extends the shorter owner to match the longer one.
4. The result is guaranteed to come from x.