I see that the FFM100 has a configuration called "Sensor:1-Phase"; what is this option used for?

The FFM100 is compatible with single phase, single pulse sensors for flow measurements such as water and other non-lubricated fluids.
This function allows the user to select specific fluid sensors that may provide a higher accuracy for a desired flow range such as water-mater output measurements or fuel transfer measurements.

Maretron has tested the GEM RFO single phase sensor and found this to be a very flexible flow sensor to use for this purpose.
The documentation for the GEMS RFO sensor includes a detailed pulses per liter chart which can help determine the correct K-Factor configuration needed for configuring the FFM100 to work with this sensor.

You can find data about the GEMS RFO sensor data and its flow accuracy at http://www.gemssensors.com/

Figure 1 shows how to connect a standard three wire fluid flow sensor to the FFM100 connection terminals. Terminals 4,5,6,10,11,12 are not used.

* Note: A Single Phase FFM100 configuration with two individual sensors will transmit two separate fuel rates over the NMEA2000 network. Depending on the displays capabilities you may need to set the FFM100 channel#1 as reverse flow in order for the display to calculate an engines fuel burn with a certain degree of accuracy. Review the aftermarket flow sensor’s datasheet for more information.