I am pretty sure that is a normal "car" style bendix starter. In which case, I am not sure what would happen with an ohmmeter touched to it. What it is, is that you should only be checking for voltage. Voltage (battery voltage, 12 volts or so, maybe down to 10 when cranking) should be there only with the starter engaged, otherwise it should be zero volts. The S probably stands to starter or solenoid, it is just the power needed to pull in the solenoid, the solenoid pushes out the bendix drive and also sends power to the starter motor itself.