What is ACR Slot used for?

Audio Communications Riser - was meant to be a cheap interface to plug in an ACR modem or expand upon your built-in sound

What is an ACR slot for? Roads? Where We're Going, We Don't Need Roads! That's the only thing I could find for ACR related to computers

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acrylic box with slot Audio Communications Riser - was meant to be a cheap interface to plug in an ACR modem or expand upon your built-in sound