The external I2S device requirements (codec/DAC). In general these devices need a master clock (usually at the rate 256 × FS) to perform internal and sampling operations.
AN2739的标题是《How to use the high-density STM32F103xx microcontroller to play audio files with an external I2S audio codec》,从这个标题可以知道它是介绍如何与外部I2S的Codec接口,所以6楼引用的那句话中当然要在扩号中标明(codec/DAC),用以具体指明。