AD9694: Input Common Mode

I did not understand the introduction of Input Common Mode in the Datasheet.

My question is:

AC coupling input, which registers need to be configured? How to configure it?

How to configure the registers 0x18E0, 0x18E1, 0x18E2? What is the meaning of each Bit?

The External VCM Buffer in register 0x18E3 BIT6 is enabled to output VCM voltage to the pin, If I configure AC coupling, how is this bit configured? External VCM Buffer referred to in Datasheet 57 When is the Buffer enabled? When is the buffer disabled?

Above is my question, hope to get your answer, thank you!