Vref is the reference voltage, v is the measured voltage, FF is the feed-forward value that we may want to add to the output of the voltage controller. Considering the PI controller output u_pi=(Kp+(Ki/s))*(Vref-v) we can add a feed-forward precontrol term to u_pi such that the controller output is u=u_pi+FF. Reset input is used if we want to reset the controller, such that when we switch between Buck and Boost modes.
More insights about this block can be found in the documentation:
I hope this helps.
