The full face tempered glass is a full coverage hardened glass, specially designed to protect your phone's screen from scratches and other damage. In case of dropping your phone, it provides additional protection, although it does not guarantee that the screen will not crack. It does not affect the functionality of the screen.
The black border covers the entire surface of the screen, including the edges. The special treatment of the glass makes the screen visible only from the front, blocking visibility from any angle. This protection is especially useful for people who often use public transportation and want to keep their screen private.
On the other hand, it is not suitable for gaming or watching videos, as it may darken the screen.