I'm struggling to understand the difference between "counterpart prices", "transaction prices" and "best prices" to do my taxes. The discrepancy between these three modes are extreme. If I use the "best prices" option, my UDST gain is close to zero, but if I choose the "counterpart prices", I have a huge negative gain in USDT (equivalent to 1000 real USD). I've only used USDT as a means to buy and sell other cryptos, never holding USDT for more than a couple of minutes. And I haven't done many trades either. So why is the gain calculation so different between the modes?

And what is the real gain I should use for my taxes? My realized gain is 25% lower with "counterpart prices" compared to "best prices"