Here is what Kevin Deal of Upscale Audio has to say about it. He has bought, sold, tested and listened to more tubes than your average Audiogoner.
https://www.upscaleaudio.com/pages/grading-matching
Driver / Buffer grade -
Note: There is nothing wrong with these tubes in many applications and they are an opportunity for you to save money!
The tubes that are low in noise, but not lowest in microphony. There are some tubes that will never be low in microphonics... this is by design. Usually these tubes are used in a circuit where they don’t need to be low in microphony, such as the driver stage of an amplifier, or an output buffer in a DAC or CD player. In these circuits, purchasing a low microphony tube will not yield any performance benefits, so take advantage of the lower price, and take your sweety out for a nice meal.
Please note that with some tubes types, almost 90% will be this grade. Count on us to walk you through and get you the right stuff.
Gold - This is a tube that is low in microphony and has excellent noise and triode balance measurements. Typically triode balance for Gold Graded tubes will be between 15% and 20%, but may be higher depending on available stock. Perfect for preamps, phono stages, and the input stage of power amplifiers. It can’t get much better, unless you go to Platinum Grade or Kevin’s Stash.
Platinum - Sometimes you just want the cream of the crop... Whether it’s personal preference or a difficult circuit, our customers demand the absolute best. So for this customer, we offer Platinum grade. The difference between Gold Grade and Platinum Grade is a measurable difference in triode balance. Small signal tubes such as 6922, 12AX7, 12AT7, and others are like two tubes in one glass envelope. That is why they are called "Dual Triode." Triode balance in Platinum Graded tubes is typically 10% or tighter, but may be higher depending on available stocks. They will also measure at the tippy top for transconductance.