Fully proportional would imply a PID loop of some kind, IOW the process would have feedback or closed loop.
Proportional - Integral - Derivative.
The PID controller calculates an error value as the difference between a measured process variable and a desired set point.
The controller attempts to correct the error by adjusting the process control inputs.
An example of an open loop system would be similar to modern electronic heating thermostats, when the set point is neared, the process progressively cycles until the setpoint is reached, this avoids large overshoots that you can have with a simple mercury and bi-metal strip.
Max.