IN is the switch control pin and is a logic input. The analog switch paths are bi-directional, and pass signals the same in either direction. Since the MAX4644 is a single pole double-throw (SPDT) switch it can be configured as a 1:2 "demultiplexer" where COM is the input and NC and NO are the outputs. Likewise it can be configured as a 2:1 "multiplexer" where NC and NO are the inputs and COM is the output.