When you initiate an exchange, you are quoted a rate. Whether that rate is fixed or floating determines whether the amount you receive is locked at that quote or calculated at the moment your deposit confirms.
The difference matters, but it is smaller than most people assume — and the framing of "fixed is safer" is almost always wrong.
What actually moves the rate
For most asset pairs, the price movement between initiating an exchange and the deposit confirming is less than 0.5%. Bitcoin on a normal day moves roughly 0.02–0.08% per minute. Unless you are exchanging during an extreme volatility event — a major liquidation cascade, a significant news event — the floating rate you receive will be very close to the quoted rate.
The floating rate is the market rate. The fixed rate is the market rate plus a premium for the certainty.
Fixed: rate locked at quote · costs 0.5–1.2% more in spread
For small exchanges, this distinction is academic. For larger exchanges — where you are moving meaningful value — the calculation is worth doing. The fixed rate premium is predictable. The floating rate variance is historical. In most conditions, floating costs less in total.