Some currencies are multi-chain assets. Unlike other cryptocurrencies, such coins can be transferred using different networks/layers. For example, USDT is supported not only by the ERC20 layer but also by TRON, Solana, BEP20 and etc.
It gives you an option to choose the network with a lower fee, the network you used for transactions before, or the network which is available on both sides.
Note that the wallet from which you withdraw/deposit and the wallet to which you deposit/withdraw must be supported by the same network.
If you do the transaction based on the ERC20 blockchain, you use the USDT ERC20 wallet address.
It means that if on the other side you choose the ERC20 layer, at STEX you also need to choose the ERC20 layer.
Then the transaction will take place in the ERC20 network.
Transactions from different networks can't be credited automatically. Sometimes it can cause a loss of your coins.
Pay attention to the warnings you see while depositing/withdrawing coins.
Note: if the network is disabled due to maintenance you can see a warning message under the layer you choose. Please be attentive.