May I ask what is the difference between USD and USDET ?Is there any policy or restriction to Usdet ?
USDET is the IOU tokens that BTC-e has issued.
As per their official announcements, these tokens can be traded freely. You should have tokens in your account right now if you had any sort of balance at BTC-e and didn't cash out early and get the 55% in liquid cryptocurrencies. You can either wait for BTC-e to redeem these tokens eventually or trade them now.
Personally i'd lean towards the first option. BTC-e has proven that they are here to stay and 100% legit, because if they weren't then they wouldn't have done such extensive recovery to their business and even reimburse anything at all.