I see - but does it mean that trading is different - do we have to go through another verufication to trade usd/btc?
Withdrawing dollars tends to require a lot of verification. Holding them in an exchange is a different matter, if you intend to end up with Bitcoin in the end.
Trading USDT just requires going on an altcoin exchange like Bittrex, and withdrawing them to a USDT wallet.
The difference is that you can sovereignty over the funds like you do with cryptocurrencies, without going through the verification process of real fiat.
However, I wouldn't trust USDT because the price fluctuates a lot more than you would imagine.