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A floating rate is calculated when your deposit confirms. Between clicking and confirming, the market moves, and you receive whatever the pair is worth at that later moment — better or worse.
A fixed rate locks the number for a short window, typically ten to thirty minutes, and the provider charges a slightly wider spread to carry that risk. You are buying certainty, and on a calm day it is a small premium for very little benefit.
The decision is really about the deposit chain. If you are depositing Bitcoin and the network is busy, your confirmation could be half an hour away and a fixed rate is worth paying for. If you are depositing on Solana or Tron, confirmation is near-instant and floating rarely surprises anyone.
Either way, the quote screen labels which is which before you commit. Read the label, not the headline number.