Prove that checking if context free grammar generates word with all symbols is NP-complete - unclear statement about encoing probem in exercise
What the remark is trying to tell you, I think, is that the alphabet of each different grammar $G$ can grow and is not fixed.

For instance. say that you have a language $L$ that you want to reduce to your language, let´s call it $L'$, then, you are allowed to make the alphabet of the different context free grammars inside $L'$ grow as the instances inside $L$ grow. For a bigger instance a bigger alphabet