If the use of this feature is properly controlled, the address can contain information about the building, department, room, machine, wiring circuit, or owner's telephone number. When accurate, such information can speed problem determination. In normal operation, an Ethernet adapter will receive only frames with a destination address that matches its unique address, or destination addresses that represent a multicast message. If this poses a security problem, a new generation of smart hub devices can filter out all frames with private destination addresses belonging to another station. The textbook definitions of Ethernet have little to do with current practice.
Ethernet is supposed to be a single common medium with multiple connections.
That may be true for older installations and laboratory environments. However, new desktop installations bring Ethernet to the desktop over phone wire and frequently build the spine using fiber optics. The connection to the rest of the LAN involves active electronics in the hub. In current use, this is done with a repeater that copies every bit and propagates collisions. Only multicast messages and private messages specifically addressed to the PC are forwarded to the desktop.