Linked below are several posts that have more information about controlling 393 motor connected to a 3-wire legacy port on V5 brain (obviously, via MC29 motor controller).
The keyword to know is vex::pwm_out
vex::pwm_out finger( Brain.ThreeWirePort.A );
or
vex::pwm_out finger = pwm_out( Brain.ThreeWirePort.A );
configures pwm_out on legacy port A.
current is limited to 2A. Voltage is 5V. There is no PTC, the voltage regulator chip will shutdown if too much current is pulled. The VCS/VEXcode motor29 class has some slew rate code to help with current transients, the motor_victor and pwm_out classes drive without that. The V5 supports the old motors primarily for classroom use with lights loads.
In simple terms the value is controlling motor voltage, more accurately the pwm (pulse width modulation) control of the motor.
several years ago we did lots of testing and analysis of 393 motors.
here are some relevant threads, many links are now broken due to forum upgrade.