Don't laugh at me—I had to learn the hard way, too.
Today's topic: isolated margin vs. cross margin
In cross margin mode, the wallet balance of the same margin asset in your futures account is shared as collateral for these positions, so unrealized losses can draw on your entire balance.
If you choose cross margin, don't transfer all your funds into your futures account—only transfer the amount you plan to trade with.
Coin in the example: $BTC is quoted at 81600.48 (24h -0.97%).