What does one lifetime name shared by two parameters and a return value guarantee?

What does one lifetime name shared by two parameters and a return value guarantee?

1. Both input borrows are valid for a caller-chosen region in which the result may be used.
2. Both owners must be created and dropped in the same lexical block.
3. The function extends the shorter owner to match the longer owner.
4. The result is always borrowed from the first parameter.