This pattern expresses that while you are not sure about or cannot vouch for the first thing, the second thing is definitely true or different. It's like saying 'I don't know about A, but B' or 'A might be so, but B is certainly the case.' The first part often presents an extreme or hypothetical example, and the second states a contrasting fact. It is a formal and somewhat literary expression, so you will mostly see it in writing or formal speech, not in everyday conversation. For example, you might say 'I don't know about other countries, but in Japan, it's customary to take off shoes indoors.' This pattern highlights the contrast between what you don't know and what you do know. Note that the first part is often introduced with は or なら, and the second part typically ends with a negative statement or a contrasting fact.