A potential which can be floating, being a reference voltage in schematic.
GND must only be used when potential is intended to be connected to CBN.

Common Bonding Network, (Chassis/Frame).
CBN must* be connected to earth using a Protective Earth cable (yellow/green standard cable).
CBN should* in addition be connected to earth through low impedance path for EMC purpose.
* In some specific cases of system being used in high voltage floating applications, an internal CBN can be connected to a live voltage, being included in another external and safe earthed CBN (Matryoshka doll).

Earth, or protective earth.
It is the key element of safety and lightning protection.