How can char[] represent a UTF-8 string?

In C11, a new string literal was added with the prefix u8. This represents an array of chars with the text encoded as UTF-8. How is this even possible? Isn't a normal char signed? Meaning it has one bit less of information to use because of the sign bit? My logic would depict that a string of UTF-8 text would need to be an array of unsigned chars.