This option indicates the medium through which the heartbeat packets between the two units are exchanged (failover option). Choose LAN to exchange the heartbeat packets over the LAN interface. Choose Serial to exchange the heartbeat packets over the Serial interface. It is necessary to have a null modem cable for connecting the serial ports between the two units.
Also, if an unused interface is not used on the FatPipe, that interface can be used to interconnect the FatPipe devices. For example if WAN 3/ETH3 is not used, then that interface can be selected in the unit failover portion of the GUI to be used for the heartbeat (contact FatPipe Support, as this is done through the Root console).