ISS is the simulator that represents the instruction set.
IIS is communication that is mainly used for audio transmission, and most of the time you see one side sending, the other side receiving, in a one-way form (I always feel like I've seen two-way communication somewhere.
Typically, audio-related communication on the development board uses IIS, which is sent from the multimedia CPU to the DAC chip.
Its communication form is very similar to SPI, but it generally has two clock lines due to the characteristics of sound data.