What's the point of RSD or %CV...?

RSD and %CV are the same thing. They both express the variation (standard deviation) as a % of the mean.

what this tells me that as the mean goes up, then does the variation. And if the variation goes up, then so does the uncertainty in the measurement system at the higher values.

Also, RSD only tells us a value of the standard deviation, it doesnot tell us where that variation is coming from…person, assay equipment, sample, or an interaction of an of these.

Wouldn’t a better approach to assay validation be the use of a Gage R&R study?