I know that some TPGs will (or at least did in the past) slab bulk submission coins as "Brilliant Uncirculated" if they don't make a specific grade specified by the submitter. I have a 1/10 AGE in an NGC slab like this.
Isn't BU an archaic grade ? A name subject to interpretation ? Styx 'n' Stones may rock my bones, but names won't change the grade
Okay, I tried to put together a grading set but it is tough to get all the grades for one year/mm. This isn't perfect and many of the coins are toned, but it should do the job. Here is my grading set for 1940-D Jefferson Nickels. BU (MS63): Flat luster and major marks, specifically on forehead and in field below MONTICELLO CH BU (MS64): Ignore the counting wheel damage, otherwise better luster, but too many small marks in the focal areas for a gem grade. GEM BU (MS65): Gem grade luster and an appropriate number of small marks. Premium GEM BU (MS66): Gem Grade luster with extremely clean surfaces, a few minor ticks. Ultra GEM (MS67): Booming Luster, registry quality coin in all aspects of grading including eye appeal.
For a more cynical interpretation, for some dealers, the term “BU” met “Been Used.” Palming off shiny silver coins that had anything from a light rub to polishing and whizzing was the order of the day in the 1960s and ‘70s.
Choice BU is what the overstuffed, balding, sweaty, dirty-fingernailed dealers (Katz Coins type) sell their raw coins at. AU is what they buy back the same coin for.