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Dolby AC-4 is the newest codec for audio offered from Dolby. It has a lot of features and its specification is quite long. What you are referring to is probably AC-4 IMS considered as AC-4 Immersive Stereo, which is two channel standard. This is a solution dedicated for mobile devices to cover its headphone playback or build in loudspeakers and to efficiently transmit Dolby Atmos experience.
Here is some AC-4 documentation that covers some details about AC-4 https://professional.dolby.com/siteassets/technologies/dolbt_atmos_ac-4_whitepaper.pdf