Normally, an HDMI signal is transmitted from a source to an HDMI input. A television, for example, has such HDMI inputs. This is how the picture and sound from a source device are sent to the display.  

ARC or eARC is a return channel for an audio signal travelling in the opposite direction, i.e. from the display to an external device. The audio return channel (HDMI ARC or HDMI eARC) is only used if the sound system is connected to the TV and

• the TV itself is providing the signal (for example, from an app or TV tuner)
• a playback device is connected directly to the TV and the audio signal is being relayed

Dolby Atmos Scenarios:

The prerequisite is always that an Atmos-compatible soundbar is connected to the TV.

Playback device connected to the TV:

If Dolby Atmos is being played from an Xbox, PC, Blu-ray player or an Apple TV 4K and needs to be routed through the TV, HDMI-eARC on the TV is therefore required.

External device connected to the sound system:

If the external device is connected directly to the sound system’s HDMI input, HDMI-eARC on the TV is not required. In this case, the standard HDMI connection is used, which supports all audio formats (including Dolby Atmos). Many soundbars do not have such an HDMI input, only an HDMI-ARC or HDMI-eARC port.

Internal TV app and soundbar, no external player

Even if no external player is used and the TV supports Dolby Atmos, Dolby Atmos can be played via a compatible soundbar. In most device combinations, eARC is not required for this; ARC is sufficient.