What is the explanation of the principle "Whatever is open to possibility cannot be used as proof", with examples?

The meaning of the principle is that an evidence which admits of more than one interpretation, and none of them is preferred, cannot be used as an argument against an opponent or as a definitive text on an issue. What they mean by the invalidation of its use as evidence is that it cannot be used to determine the intended interpretation, not that it is entirely invalidated. The intended meaning of "possibility" (or "probability") here is a strong possibility supported by contextual indicators, not just any possibility; otherwise, the use of all evidence would be nullified. Examples of this include the Almighty's saying: "And divorced women shall wait concerning themselves for three periods (Quru’)" and the Prophet's (peace be upon him) saying: "The two parties in a transaction have the option (to annul it) as long as they have not separated, or until they separate."