At one time the “R” terminal was used to bypass the ballast resistor and supply full voltage to the coil at startup on gasoline engines but the only time we use the R terminal on
diesel engines is if there’s a shutdown solenoid with a 3 wire hookup. Typically there’s a momentary “pull in” winding and once the
engine is started, the hold in coil takes over and avoids the common
overheating and burnout of the pull in winding, the third wire is usually a -ve. R is frequently used on gensets but is seen much less often these days ( there are several other ways to get momentary pull in
power to the stop solenoid).