I would say that BU should mean MS63 or better Uncirculated and Brilliant
Needless to say that a several vendors call an MS60 a BU which it is not
It should be about the same as Choice Uncirculated or MS63 or better
FDC Fleur de Coin is another abused term . It should be MS65 or better but is abused from MS60 onwards
In Europe the most abused term is probably Bankfresh which means in condition as received by the bank which is MS60 and upwards
Mintfresh is supposed to be used for MS63 or better
But since hardly anybody can grade the terms really mean very little