Tube Test Results

I've read descriptions of tubes for sale here claiming that claim the tubes for sale "Good" or "Excellent" on their teting machines. What does this mean?

If a tube is not "Excellent" does that mean that their is not as much life left but, sound quality is not affected or, tube life is deteriorating and sound is too.

I guess my ultimate question is, do tubes burn out like a light bulb - running strong until the filament burns up or, do they die like a battery - less and less energy over time?

Talking about specifically 6SN7GTs as an example - if that matters.

thanx