XREX now supports both TRC20-USDT and ERC20-USDT.
TRC20-USDT refers to USDT issued on the TRON network, ERC20-USDT refers to USDT issued by Tether on the Ethereum network. The USDT issued on TRC20 and ERC20 are both identical, however fees to transfer this USDT can often be cheaper on the Tron network using TRC20.
Deposit addresses for these are always Ethereum addresses and these deposits / withdrawal transactions occur on the Ethereum network. The protocol of USDT-ERC20 is the ERC20 protocol.