The invoice does mention the replacement of the helicoid in the lens, which would be a more invasive repair and likely what the 'rank C' is referring to.
I was under the impression that they were going to replace the front element. But for some reason they did not. I was hoping that 'rank C' was a specific code for lens element replacement, but apparently not. It could be they either thought the problem I reported (some small damage spots on the front element from an impact) was not significant enough to necessitate repair or they ignored the repair. I'll have to call and find out which. $600+ seems a little high for just replacing the helicoid. (Mind you, I have no idea what's involved with that.)