On RC4Magic Series 3 and RC4Magic-900 DMXio transceivers, the Auto/RX/TX button cycles through modes.  Each device can operate as either a receiver (RX) or a transmitter (TX).  In Auto Mode, which is the default setting, the DMXio automatically determines which mode it should be in.  Connect DMX data to a DMXio and it will be a transmitter.  Any other DMXio will find the transmitted signal and become a receiver.  It is rare that a device would need to be forced into receiver or transmitter mode, but some system operators may prefer to eliminate any possibility of a device changing it’s role within an installation.