The difference between Bases A and B is the quantity in the packet. The composition is the same but one contains more to impart different levels of sweetness.
Base B has a higher weight than Base A. Since both are intended to make the same 1.125 L volume, Base B is designed to make a sweeter liqueur drink than Base A. See individual flavouring, for which base is recommended to be used with it. Also will make a final ABV lower than base A.