Acoustic systems with coaxial input
— a variation of the S/P-DIF standard that uses an electrical cable with an RCA plug. Like another version of this standard — optical — provides a digital signal format and allows you to transmit multi-channel audio through a single connector. At the same time, the electric cable is somewhat more susceptible to interference than optical fiber, but it is more durable and does not require special delicacy in handling.
We emphasize that you should not confuse the coaxial input with analogue RCA (see the relevant paragraph): despite the same connectors, these are fundamentally different interfaces.