You can refer to the documentation for the "Synchronous Machine pu Fundamental," which mentions that the "Vf" input is normalized with respect to the nominal field voltage so that a 1 pu input produces a 1 pu stator voltage at no load. This is why there is a "Gain Block." Here is the link to the documentation:
Additionally, for more information on the "pu units system", refer to the following link:
Hope this helps!
