When someone says:
Should you require X, you are to advise us by date Y
— does that also mean that I am to advise them even if I do not require X? Or does that only say that advising them is only needed if I require X?
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Technically, the example sentence does not say anything about a situation in which "you" do not "require X". This implies that you do not need to do anything if you do not require X. It also implies that your counter-party is assuming that you do not require X, unless you say otherwise.
This kind of contract clause is often used when your counter-party needs to take certain actions close to date Y, based on whether or not you need X. The sentence does not explicitly say what will happen if you need X, but do not tell your counter-party by date Y. Presumably, you will lose any right or expectation to X if you fail to provide timely notice. There might also be other consequences.
A lawyer who is both fluent in English and practices law in the jurisdiction where the contract would be adjudicated can provide more information about the consequences of the example sentence.