Class N was introduced in the 2016 edition of NFPA 72 to define performance requirements for Ethernet (network) based fire alarm equipment. Wiring for Ethernet devices is typically home run rather than looped (like traditional fire alarm equipment), making the supervision requirements more challenging.
A key requirement for Class N circuits is a redundant path from the control equipment to the network equipment. The connection from the control equipment must be at least two separate connections to the network equipment (typically a switch). Since each individual endpoint device on an Ethernet network is home run to a switch, redundant cabling to each Ethernet device is not required. The failure of a single device or wiring to that device will only affect that device.
Class N circuits may also be used to communicate with non-Ethernet devices as well, for example, a Class N circuit could be used to connect a control panel to an audio amplifier driving speakers. The connection from the amplifier to the speakers would then be Class A, B, or X circuits.