Select, Gem, Choice BU

I hate those terms, they're generally used as a cop-out to assigning a real grade. In general, I consider them:

choice BU - MS63 or lower
select BU - MS64
gem BU - MS65
ultra gem BU - MS66 up

Others may have different opinions which is why so many sellers use these terms.

I have this in my reference:
MS-70: perfect
MS-67: gem
MS-65: choice
MS-63: select
MS-60: uncirculated

That's the correct progression, but "grade creep" has lowered the industry standard somewhat. Sellers use select for coins MS60~62, Choice MS63~64, and Gem is MS65 and up. The MS 67~70 grades are assigned multiple glowing adjectives added to "gem" (ultra, mega, supercalifragilisticexpealodocious, etc.)

As noted those expressions or terminologies are used by many dealers to basically attempt to sell better. For many individuals that are not sure of the MS-62, -64, -66 type of stuff, a simpler Choice, BU, etc. type of terminology soulds much better. Select, choice, Brilliant, Bright, Gem, Ultra anything, full something or other and on and on are really helpfull in sales for many dealers. Attempting to explain conditions to each prospective purchaser of you merchandise, let the words speak for you.
Ever watch TV. Listen to their explanations of everything. Ever hear them say "It's OK I guess" or "Kind of decent" etc. No, everything is fantastic, wonderful, great, beautiful, gorgeous, etc.
Same with dealers selling coins you know. Possibly some coin dealers also are used car salesmen.